This is Ad Skipper for YT Extension.
Doesn't seem to be working and triggers yt's "Ad blockers are not allowed", also doesn't specify how to replace ads with gifs. Two extra stars for effort.
https://youtube-skins.com/
Natalius
Saurav Loach
Happy Fone
Avinash Sakhreliya
YT-LightBlock
Zack Sempsrott
Taha Boulehmi
mauricebuckner966
Anuj Gupta
shreyadahal
https://blockforytb.com/