Custom GPTs, Gemini Gems, and prompt libraries all promise the same thing: stop retyping the same instructions and get consistent results. But they solve that problem in very different ways, and picking the wrong one leaves you fighting your own tools. This guide explains what each actually is, where each wins, and how to decide based on how you really work.
The short version: a custom GPT or a Gemini Gem is a saved bot that lives inside one platform, while a prompt library is a set of reusable prompts you run in any assistant. They aren't rivals so much as different jobs. Knowing which job you have makes the choice obvious.
Key Takeaways
- A custom GPT or Gemini Gem is a saved bot with baked-in instructions that lives inside one platform; a prompt library is reusable prompts you run anywhere.
- Custom GPTs and Gems shine for a repeated, complex workflow inside one ecosystem.
- A prompt library wins for fast reuse across ChatGPT, Claude, Gemini, and Perplexity.
- They're complementary: build a Gem or GPT for your deep one-tool workflow, and keep a library for everything you run across tools.
What is a custom GPT or a Gemini Gem?
A custom GPT (in ChatGPT) or a Gemini Gem (in Google's Gemini) is a saved assistant you configure once. You give it a name, a set of standing instructions, and sometimes reference files or knowledge it should draw on. After that, you open the bot and start chatting, and it already behaves the way you set it up. No need to paste your instructions each time, because they're baked into the bot itself.
People took to this fast. Within two months of OpenAI launching the feature, users had built roughly 3 million custom GPTs (TechCrunch, 2024). Google followed with Gems, which let users build their own custom AI experts and rolled the feature out across more than 150 countries (Google, 2024). The appeal is clear: package a complex setup once, then reuse it without rebuilding it every session.
The catch is where they live. A custom GPT only runs inside ChatGPT. A Gem only runs inside Gemini. The setup is genuinely useful, but it's locked to the ecosystem you built it in. Move to another assistant and you start over.
What is a prompt library, and how is it different?
A prompt library is a collection of saved, reusable prompts you can run in any assistant. Instead of a bot that holds your instructions, you keep the instructions themselves as text you can drop into whatever chat box you're using. The same prompt works in ChatGPT one minute and Claude the next.
The difference comes down to where the logic lives. A custom GPT or Gem holds your instructions inside a bot on one platform. A library holds your instructions as portable prompts you carry between platforms. One is a configured destination you go to; the other is a toolkit you bring with you. If the concept is new, what a prompt library is walks through the basics.
That portability matters because most people don't stay in one assistant. They draft in one, fact-check in another, and code in a third. With ChatGPT alone reaching about 900 million weekly active users in early 2026 (TechCrunch, 2026), and strong tools from Google, Anthropic, and Perplexity all in daily use, betting your whole workflow on one ecosystem is a real constraint. A library sidesteps that by staying tool-agnostic.
How do custom GPTs, Gems, and prompt libraries compare?
Here's how the two approaches stack up on the things that actually shape daily use. Both are good tools. They're built for different jobs.
| Custom GPTs / Gemini Gems | Prompt library | |
|---|---|---|
| Setup effort | Higher: name, instructions, knowledge files | Lower: save a prompt as text |
| Works across tools | No, locked to one platform | Yes, runs in any assistant |
| Sharing | Share within the platform's ecosystem | Share prompt text anywhere |
| Best for | A repeated, complex workflow in one tool | Fast reuse of many prompts across tools |
| Cost | Often needs a paid tier to build | Free options exist |
The pattern is straightforward. A custom GPT or Gem invests more setup up front to hold a richer, more complex configuration in one place. A library trades that depth for breadth: less to configure per prompt, but it goes everywhere. Neither is better in the abstract. The right one depends on whether your need is deep-in-one-tool or wide-across-tools.
A prompt library that follows you across every AI
Promptly keeps your prompts one click away inside ChatGPT, Claude, Gemini, Perplexity, and Deepseek, free to start.
When does a custom GPT or Gem win?
Custom GPTs and Gems are the right call when you have one repeated, complex workflow that lives inside a single ecosystem. The clearest signs:
- The setup is heavy. Long standing instructions, a defined persona, maybe reference documents the bot should always consult. Packaging all of that into a bot once beats pasting it every session.
- You're committed to one platform. If your work, your data, and your team already sit inside ChatGPT or Gemini, a bot native to that platform fits cleanly with no portability concerns.
- You want a sharable, named assistant. A custom GPT or Gem you can hand to a teammate inside the same ecosystem is a tidy way to distribute a configured workflow.
Think of a support assistant trained on your help docs, or a brand-voice editor loaded with your style guide. The configuration is the value, and it's worth the setup because you'll lean on that one bot constantly. For that kind of deep, single-tool use, a custom GPT or Gem earns its place.
When does a prompt library win?
A prompt library wins when you reuse many different prompts and you don't want to be tied to one assistant. The signs here are the mirror image:
- You use several tools. You move between ChatGPT, Claude, Gemini, and Perplexity depending on the task. A library runs the same prompts in all of them, so you stay consistent without rebuilding anything. More on this in managing prompts across AI tools.
- Your prompts are many and varied. A blog outline, a code review, a meeting summary, a cold email. These don't each justify their own bot. Saving them as quick-to-grab prompts is far lighter than configuring dozens of GPTs.
- Speed of reuse matters more than depth of setup. You want to drop a known-good prompt into the chat and go, not open a separate destination first.
This is where being tool-agnostic pays off. You're not betting on one company's roadmap, and you're not maintaining a copy of your workflow per platform. For a roundup of how libraries compare to other management approaches, see the best AI prompt manager tools.
Where does Promptly fit?
Promptly is a free browser extension that gives you a cross-tool prompt library right in your browser. Your prompts sit next to the chat box, so reuse costs a click instead of a tab switch, and the same library runs across ChatGPT, Claude, Gemini, Perplexity, and Deepseek. It can also move a conversation between those assistants, so you're not boxed into wherever you started.
That places Promptly squarely in the "wide across tools" job. It doesn't replace a custom GPT or a Gem, and it isn't trying to. If you've built a deep bot for one heavy workflow, keep it. Promptly handles the dozens of everyday prompts you run everywhere else, the ones that would be a chore to turn into individual bots. Because it's free, you can build the habit before deciding it's worth anything to you.
The honest framing: if your entire need is one complex workflow inside a single platform, a custom GPT or Gem may be all you want, and that's fine. The more tools you touch and the more prompts you reuse, the more a portable library earns its keep alongside any bots you've built.
Which should you pick?
Match the tool to the shape of your work, not to which one sounds more advanced.
You have one heavy, repeated workflow in one tool. Build a custom GPT or a Gemini Gem. The setup investment pays off because you'll use that single configured bot over and over inside the ecosystem you already live in.
You reuse many prompts across several assistants. Use a prompt library. The portability and the low cost per prompt are exactly what a wide, cross-tool habit needs, and a browser extension keeps it one click from the chat box.
You do both. Use both. Keep your deep bots for the workflows that justify them, and keep a library for everything you run across tools. They cover different ground, so there's no conflict in running them side by side.
Still weighing whether any dedicated tool is worth the effort over copy-paste? Read is a prompt manager worth it? for an honest cost-benefit look before you commit.
Frequently asked questions
What's the difference between a custom GPT and a prompt library?
A custom GPT is a saved bot inside ChatGPT with your instructions baked in, so you open it and chat without re-pasting setup. A prompt library is a set of reusable prompts you keep as text and run in any assistant. The GPT holds your logic on one platform; the library lets you carry your logic between platforms like ChatGPT, Claude, Gemini, and Perplexity.
Can I use a custom GPT or Gemini Gem in other AI tools?
No. A custom GPT only runs inside ChatGPT, and a Gemini Gem only runs inside Gemini. Each is locked to the ecosystem you built it in. If you want the same instructions available across several assistants, a prompt library is the better fit because the prompts are portable text you can run anywhere.
Are custom GPTs, Gems, and prompt libraries free?
It varies. Building custom GPTs or Gems often requires a paid tier of the platform, though using ones others made can be free. Prompt libraries have free options. Promptly is a free browser extension that stores your library and runs prompts across ChatGPT, Claude, Gemini, Perplexity, and Deepseek with no paid gate to start.
Should I use a prompt library or build a custom GPT?
Build a custom GPT or Gem when you have one complex, repeated workflow inside a single platform and the heavy setup pays off. Choose a prompt library when you reuse many different prompts across several assistants and want them portable. Many people do both: deep bots for one tool, a library for everything cross-tool.
Do custom GPTs and prompt libraries compete with each other?
Not really. They're complementary. A custom GPT or Gem packages a deep, single-tool workflow into a configured bot, while a prompt library keeps your everyday prompts reusable across every assistant you touch. Running both is common: use the bot for the workflow that justifies its setup, and the library for the dozens of prompts you'd never turn into individual bots.
Sources
- TechCrunch. OpenAI's chatbot store is filling up with spam (2024). https://techcrunch.com/2024/03/20/openais-chatbot-store-is-filling-up-with-spam/, retrieved 2026-06-16.
- Google. Gemini update, August 2024 (2024). https://blog.google/products-and-platforms/products/gemini/google-gemini-update-august-2024/, retrieved 2026-06-16.
- TechCrunch. ChatGPT reaches 900M weekly active users (2026). https://techcrunch.com/2026/02/27/chatgpt-reaches-900m-weekly-active-users, retrieved 2026-06-16.
- Hero image: James Wheeler via Pexels.