There's nothing quite as much fun as learning something new at three o'clock in the morning.
For most of these I just played around with the colour table in a hex editor, I also tried messing with the Graphic Control Extension block to play with transparency but I couldn't see any effect, I think I need to learn more about how it works. Another thing I did that had quite an effect was to add dithering and strange colour palettes in GIMP.
This is the original, just as a reference.
And these are the glitched versions.
I think that last one is my favorite.