If you want to do animation I'd get either Synfig or Blender if you want something free, or Flash is you want to pay.
Gimp is good for drawing frames and making gifs (unless you want to mas save all your frames then you can use a program like WAX or MovieMaker) but not really a movie making program. - GAP is an option, but really if you want to animate with it I'd save the frames and open in another program to add sounds or whatever you want.
There is a mass save script for saving all layers in GIMP, this is only useful in your case if after you finish a frame you put in a new project file and save as layers, otherwise you'll end up saving pieces of a frame instead of a finished frame.
viewtopic.php?f=8&t=8313&hilit=save+as+frames+scriptI also found this program I never tried it but maybe that would be helpful.
http://www.powtoon.com/blog/best-free-a ... -software/