Imgur blocks access to most VPN IP addresses so any posts using the host will result in many users being unable to see the content. Sometimes only remote viewing is affected but is also often blocked when trying to open in new tab and view at the parent site.
Consider uploading to the fediverse or using a privacy-respecting hosting service like postimages.
Blame your instance, not “the fediverse”. Sounds like your admin configured their Lemmy instance to reject GIF uploads in an attempt to save on storage space because GIF files are so large compared to modern formats like WebP, AVIF, JPEGXL, etc. But some instances do allow GIFs. I just posted this last week because the instance I am on does: https://quokk.au/static/media/posts/pt/SI/ptSIPHNTlAYA1Ri.gif