Spine generates the highest quality GIFs that are possible, but it doesn't try to optimize for a small file size.
What is your GIF dimensions? Smaller dimensions and lower frame rate are the easiest ways to make the file smaller, especially for a long GIF. Otherwise you could post process the GIF with other software to make a smaller GIF, but it will wreck the quality.
GIF is actually a very old (30+ years!) and pretty terrible format. Video is likely a better option, if possible.