Do you mean gif images from Mastodon? These images are in mp4 format and are essentially videos. Video is not displayed in Jerboa, Voyager and web interface.

So you will have to convert these videos into real gif format.

For this you can use this service

https://imgflip.com/gif-maker

Just paste a link from youtube or reddit and select the desired video length.

If I remember correctly, the resulting image is stored for several months from the last viewing.