I suppose ffmpeg is the weapon of choice but I didn't find out how to reach my goal.
ffmpeg -i input.webm -pix_fmt rgb24 output.gif
Barafu's answer is alright. But, the resulting gif may have color conversion issue as ffmpeg complains on
Incompatible pixel format 'rgb24' for codec 'gif'. Here is what I find works:
First, create PNG Palette:
ffmpeg -y -i input.webm -vf palettegen palette.png
Then, use the palette to produce gif:
ffmpeg -y -i input.webm -i palette.png -filter_complex paletteuse -r 10 output.gif
i recommend you to use a third party converting tool called DumpMedia Video Converter, it has online and offline versions, I use the online tool all the time, save time and space, but you can use the offline tool to convert in batch, means you can convert the whole playlist at one go.