I hate it, it's complicated. (Not sarcasm.) After opening the image you go to the top menu button Layer, then go to Transparency and click Add Alpha Channel. It will appear as though nothing happened so go to the Fuzzy Select Tool,
, click on the portion of the image you want transparent (needs to be a solid color) and the press delete. It should become a gray checkerboard but when it's saved as a GIF or PNG it will be transparent.