Why you should try making small gifs
It is very easy to create a very complex gif that will weight 2Mb. However, it is a challenge to make a gif < 100kB.
You should do it for the challenge, but also because smaller gif files means less bandwidth and faster loading.
Moreover, making small gifs is an exercise in conveying messages in as few bytes as possible.
One liners
Convert final color to black and white:
col.rgb = vec3(1.0 * length(col.rgb));
Reducing color detail:
col.rgb = 0.1 * floor(col.rgb * vec3(10.0));
Reducing color detail (even more):
col.rgb = 0.5 * floor(col.rgb * vec3(2.0));
Shape
It will be generally easier for gif tools to make smaller gifs when your shapes are simple. Try simplifying your objects and do not put too many wild sinusoidal curves with high periods.
Multi-pass
Edge detection could help make smaller gifs. If you draw only edges and no filling, there is less pixel information to be stored.
Image size & frame count
Of course, a smaller gif width and/or height and/or length will generally mean a smaller size in bytes. Try with less frames to see if you still like the gif.
Clip image
Sometimes, you can keep only a part of an image and keep all its meaning. For example, you could set every pixel outside of a certain circle to a black color.
Other tips
ImageMagick
If you use Linux, you might want to try using the .zip export feature in conjuction with ImageMagick.
Dithering
If you want to spend a lot of time coding, you could try learning about dithering. Search for: Dithering, Floyd Steinberg Dithering, ordered dithering.
Many dithering technique are implemented in ImageMagick, but you could try implementing them in GLSL or your favorite language (Python? C++? Javascript? Java?).