Okay I downloaded GIMP 2.6 and then the appropriate GAP version (also 2.6).
What I get from the official site is this file: "gimp-gap-2.6.0.tar.bz2"
How do I install this plug-in? I'm so frustrated. I've tried everything that I dare risk without screwing up my computer. I'm using Windows XP Home. I'm aware of the older posts about this problem but I see people talk of installers and old versions so I don't know if an old installer will work with the modern 2.6 versions. Please, someone take some time and help me before I just give up.