You can upload files. I've made one to act as a coding assistant to a little-known language that way. It seems to work OK. It makes syntax errors, but I'm not sure if that's a limitation of the Custom GPTs paradigm, or if the docs I gave it aren't any good. I know there are other services that do this kind of thing, but why bother finding one?

Can you say more about the information you uploaded for the language to be useful – for example, the language specification, or a "getting started with $LANG" manual? I'm tempted to do a GPT for the classic Macintosh toolkit, but since I'll need to do a lot of OCR I'd prefer to be intentional about what I spend my effort on.

I use it to avoid exactly that. Having to store, copy and paste various prompts over and over. I create one for each "domain" I operate with.

I have so far found them “useful” mostly for having a collection of system prompts, and for sharing silly prompts with friends. It’s basically a way to attach a prompt to a URL.

For instance I have a “Beastie Bot” that plays a game where it categorizes things as “Beasts” or “Beasties.”

I have 2 custom GPTs, 1 of which I use every day called Snarky IT Wizard. I uploaded the PDFs for my particular router firmware version so I can ask questions, I've specifically configured it to be sarcastic and insert humor into both responses and the code it generates. Honestly, using it makes my day every time.

The other I was tailoring as a dungeon master for 5th edition, I've uploaded PDFs of all the source materials I can purchase.

My only gripe with the system is that as of late the whole openai platform has been experienced issues constantly and I get errors when trying to customize or even ask it questions from time to time.

I created one for my HOA. I uploaded bylaws, rules, assignments and made instructions for using them for policy. Eg. "What parking spaces are assigned to unit 123?" So far, I'm impressed and it's useful. BUT $20/mon is a deal killer and I don't know if the GPT Store will make it more affordable for our community.