I follow this process when I record a clip of my screen using GNOME’s built in screen clipping software.

I find I get the best quality if I generate a colour palette first, as below.

ffmpeg -i input.mp4 -filter:v "fps=15,scale=640:-1:flags=lanczos,palettegen" palette.png

Next, create the GIF and reference the palette. ffmpeg options are one of the deep mysteries of the universe, but this usually works well for me.

ffmpeg -i input.mp4 -i palette.png -filter_complex "fps=15,scale=640:-1:flags=lanczos[x];[x][1:v]paletteuse" -quality:v 3 output.gif