In gimp, you have to make your animation using layers. After you save it (whatever the default gimp file format is i don't remeber), you have to go file>export as...>and make sure you save it as gif. Then a window will pop up with options. You have to make sure you check the box that says "save as animation".
Here is a simple tutorial:
http://www.gimp.org/tutorials/Simple_Animations/
Also I'm pretty sure you can open up a gif file in gimp (just not any other video file extentions unless you have a special plugin), it should just show up as separated layers. Then you just have to make sure you export it as a gif file again!
Hopefully this helps