I have project released under GPL. I have found an icon (.png) I want to use in it that also has a GPL license.

What do I have to do, if anything, to use it legally?

It's "just an icon", and while it represents a significant amount of work on someone's part, it doesn't come with any source code or embedded licenses that I can just copy and include with my project files.

I'm sure I can use it because it's GPL and my project is GPL. That's what GPL was designed for, but I don't want anyone to think I'm claiming I designed it or that I took it without giving proper credit.

I just reread the GPL, and it looks like I can do anything I want with it as long as I don't restrict anyone else from doing the same and if I were to modify it, I'd have to put a notice "somewhere" (but where would that be for an icon?) acknowledging that modification.

I don't see anything that would make me actually mention it at all in my documentation, etc. as long as I pass it along unmodified.

Best to make clear where it came from. That would ideally mean treating it as a separate piece of software. e.g. it's own subdir in your code, with the docs from whatever package you got it from.