[I'm continuing this on the clo-devel list because I think it's important for people to know how to mirror their repos - we occasionally get complaints about undermining the visibility of CL by keeping our own gitlab, so fine, let's mirror our repos on github... and keep the masters at gitlab.common-lisp.net where you can avail of our burgeoning CL-specific services that you won't find at github]

Oh, in case any wants to know, the format of the url isn't really clear. What I ended up using was ssh://rtoy@github.com/rtoy/cmucl.git, since rtoy is my github uid. I think; I didn't write it down and gitlab doesn't show it and there doesn't seem to be any way to get it.

I'm pretty sure the format is ssh://git@github.com/<account>/<project>.git and it expects to find the public SSH key in the <account>, so using <account> in place of git as the ssh username is redundant and may not work.

As in my attached image for example (a configuration which I confirm works):

ssh://git@github.com/gornskew/skewed-emacs.git