I would consider splitting the monster into sections.
This is going to get a bit off Gimp (and GAP) for animation and more to video.
GAP does use ffmpeg, but a very old version. Things change and especially ffmpeg syntax. What used to be simple is now a little more complicated but not impossible.
First ffmpeg for windows, If you go here https://ffmpeg.zeranoe.com/builds/
you can get a 64 bit static
latest version. All you really need from the zip is the single file ffmpeg.exe
You can make an animated gif straight into a video
ffmpeg -r 10 -i test.gif -vcodec libxvid -vtag xvid -q:v 2 -r 30 video-from-gif.avi
syntax:-r 10 input framerate -i input gif ..... -r 30 output framerate
In a Win7 virtual machine: http://imgur.com/Gchcrqp
Interesting you tried OpenShot, I sometimes use this in linux where it works very well. The Windows version is a beta
and a bit flaky but worth a try. Plenty of presets for youtube etc.
It might be worth looking at Avidemux which is well established, plenty of how-to's around. Load the first video and then keep appending further sections, just keep frame size/ frame rate the same for all. It will also add an audio track to the video. A final render to a more compressed format, say mp4 for youtube.
best of luck