Yup, just go image->scale image and make sure that the width is 90 or less. Then when you save it, make sure you save it as an animation (Gimp will ask you whether to save it as an animation or flatten it)
I've tried resizing them on gimp before, but they end up looking really bad for some reason. The one I have now was also a GIF at one point, but I decided that this single frame looks much better than the distorted animated one.
In addition to GIMP, you could also use http://gifmagic.com to change the size. I had some trouble resizing my Fluttershy avatar with GIMP(all layers kept showing at once), found that site and tried it out instead of spending two hours scouring through GIMP tutorials/help pages for one avatar.