Had a quick look, and I suspect this might be on the server side from Gfycat. There were a few Reddit posts about it no longer autoplaying so something may have changed in their iFrame tool/source.
The below link talks about using adding autoplay=1 into the link, but, I tried myself (multple times, with autoplay=1, autoplay=0, with nothing) and didn't get any result.
Adding in the autoplay=1 it would be something like:
But here is that pasted and it wasn't autoplaying for me
[old gfycat media embed was here once, but it's now gone]
Not sure if it is something that can be fixed from a forum end, but I did also find a script you could use in your browser with Tampermonkey or similar from Github.
Also, given someone wrote this to create a persistent Autoplay, it sort of confirmed for me that there wasn't a simpler fix.