I doubt that .gif will be supported. I would use tile sheets, so you might have a grid of 16x16 frames in a tile sheet, then you create an animated sprite from that. I haven't checked the sprite commands in detail, but I need to do this for an idea I have, so I'll check and get back to you - but I'm sure you'd be able to set the tile etc for a sprite, or at least set the UV coordinates so you can make your own tile system.
So preserve your sanity and stick to tile sheets, and I'll let you know how AppGameKit will support them soon.
Health, Ammo, and bacon and eggs!