Oops...
I installed GIMP the other day but I didn't like it because of that stupid X11 feature that made the application seem cluttered. So I uninstalled it and replaced it with the simple Paintbrush app. Then yesterday I was in my Utilities folder and I noticed X11 was there. I figured AppCleaner must've forgotten to delete that file, so I deleted it myself. Guess I shouldn't have...
I'm very new to Mac, so I don't know what X11 is for exactly. Is it a necessary thing to have? What can it be used with besides GIMP? If I need it, then I guess I'll download it from Apple's website.