I am considering switching to GIMP. I would like to know how GIMP compares to Photoshop in terms of functions overall. Are there things that Photoshop can do, that GIMP cannot?
"Overall" is not a good parameter to compare. We would need to make a table comparing specific features.
But I will give my 2 cents. Perhaps these are not a big issue. Perhaps they are.
One specific limitation is "Integration". There are competent tools for every application adobe has. Vector drawing, photo editing, video editing, compositing, printing. There are good "combos", but as a complete suite, there is not.
I personally use a bit PS but I do not often use Illustrator, but Corel Draw. I do not integrate PS with any video or compositing tool either.
- CMYK color mode. Yes, you can live without it for a lot of cases where you actually can leave the color conversion to a later step, but sometimes you cant. For video games, this is not an issue at all. The same for some special channels and preparations for print.
- Masking complex shapes, like hair. This is one thing where I personally use PS more, but for videogames, again, not a big issue.
But "Overall," I think you can do most things on Gimp or other programs for that matter, on Paint shop pro, on Affinity photo, Krita, Alpaca, etc. You need to go for the specifics to find a limitation, but I think a lot of them can be overcome.