I haven’t found any FOSS programs specifically for editing GIFs, so I’d like to know too. Kdenlive and GIMP can import and export them, but they’re definitely not designed for this purpose, the former being more of a typical video editor which seems to have no awareness of the loop points of GIFs, and the latter being a photo editor that imports the difference between every frame as a separate layer, making editing very awkward.

ezgif.com works for this purpose, but I don’t think it is FOSS.