How to compare across forks?

I’m trying to compare diff across forks
but don’t know how to do it
in github there is button for that

is gitlab support compare across forks?
if yes how to do it?
can I do it via API?

Overall, I’m not sure what you’re trying to do. Are you trying to compare commits from different forks to each other? If so that’s comparing two different repos and that’s a very tricky proposition. There are ways to do it but they are not for the faint of heart. I wouldn’t even attempt at answering that question.

However, if you’re simply wanting to compare the end point of each fork (last commit) and compare the contents of the files, just compare the content of the files using diffmerge or some such - no reason to get git involved in the middle of that. After all it’s simply a diff tool at that point.