Here are a few more tips on using Gimp 2.8 to help you get started.

To edit a .gif, download your desired. gif (either one you found or one you made), and open it in Gimp. Each frame will be a different layer. Work your way down, making layers invisible as you work through them. If you made the gif, you're probably want to delete the layers on the edges to get precisely the clip that you want. If you see that the layers are grainy and only tracking the moving parts(you'll know if you see it), that means the .gif has been optimized(to compress and save space). It it a pain in the ass to edit optimized .gifs, and they will look messed up if you try to. To fix this, go to filters->animations, and click deoptimized. It will open up a deoptimized version in a different window, and you can continue from that window.

For parts where you want to alter the image, you have to edit each frame, and if you want to add something new, select it exactly how you want it in a different window, and paste it as a new layer on top of the layer you want it in. Once you like where it is and you've edited it accordingly, duplicate that layer, if you plan on using that object again. You can then continuously copy the layer and place it on top of whatever other layers you want, and duplicating the layer as opposed to pasting the object itself puts the image in the same initial place every time, avoiding unwanted "wobble". Otherwise, just treat each layer like you would a normal photoshop. If you want to flip, rotate, or crop the gif, be sure to do it under the image dropdown, where it will edit all of the layers, and not just the individual ones. To crop, make a selection in any layer, go to image, and hit "crop to selection"

Once you're set with what you have, merge down the layers you added, and only those, so all you have left is the initial, now edited frames. Make this your last step. For this process I recommend going into the hotkeys (bottom of the edit menu), and assigning a button to "Merge Down", because you'll be using it a lot here. You can check your progress throughout by going under filters-> animations-> playback, which will bring up a little window in which you can preview your gif. Save it (use Save As in case you want to go back and change something), and there will be a few prompt windows. On the first one, be sure to check the box to export it as an animation. On the second, you don't really need to do anything, but you can change the speed and loop settings. Once you've saved it, upload it to imgur, etc., make sure it's how you wanted it, and you're good to go.