Hi elizachuvose,
I convert a video to GIF using GAP this way: Video > Split Video Into Frames > Extract Videorange. Then load a video at the top and on the right hand side click 'Videorange' . This lets you select the actual frames you want to use. This converts the frames to layers.
In GAP frames and layers mean the same I think.
To view your sequence use Filter > Animation > Playback from Gimp. You can adjust the number of frames, (deleting every second frame for example) applying filters to all frames, changing image size etc. There are a number of useful scripts to help manage frames for Gimp.
When finished playing with your sequence export as GIF or use GAP Master Videoencoder to export as video.
There are some experienced GIF makers here if you have more questions.
I understand gimp-gap maybe a bit 'buggy' on Windows but thats another story....