Stolen code in a GPL repo

I'm wondering whos responsible for this code if people start using it?

The user.

Can the people using it that think its under GPL in any way get in trouble for it or be made to remove it from their projects?

Yes, they can be sued (successfully) for copyright violation.

It’s not enough that you think you have permission from the copyright holder - you actually have to have permission. The law places the onus on the copier to seek out and get permission from the copyright holder.

In theory, someone deceived in this way could sue the repo poster(s) for misrepresentation, however, there are practical issues about finding them, having them in an accessible jurisdiction and if they are judgement proof.

Copyright law was created to protect physical books and paintings - it doesn’t really fit with digital methods of reproduction but it is the law. It doesn’t matter that complying with it can be hard bordering on impossible - comply with it you must.