You can try the GIMP. Each layer is a animation frame when you export project as a GIF.
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As a CLI there is gifsicle or imagemagick.
As a gui maybe gifcurry, tupitube or some other 2d animation tool https://en.wikipedia.org/wiki/List_of_2D_animation_software
ffmpeg is my go to for things like this but I can never remember the necessary incantation on the command line. Fortunately ChatGPT tends to get you most of the way there a lot of the time, I would normally asks something like the following and start from there:
How to convert a folder of png images into a gif using ffmpeg?
This is the guide for making the gif look good: https://blog.pkh.me/p/21-high-quality-gif-with-ffmpeg.html
Op this is the guide you need! Iโll leave my link up as comparison in quality of the guide a knowledgable human produces vs that predicted by an ml model
I use gifski
It's designed to squeeze the best quality possible out of the ancient format that is gif