Well the best place to learn is the official gimp website itself. You need to learn the basics before you even think of getting into working with your images.
then go to Youtube and check out videos uploaded. There is many good tutorials.
If you are trying to save it as a .giff extension you can do so when you save the image, I believe you can do that with any other image editor as well.
Take the time to learn Gimp and it will benefit you in the long run as opposed to learn bits & pieces.