One rule: we don't facemeet. If we do, only one of us leave alive.
It's not even close.
With the OGL, it permits limited content sharing (anything designated as Open Game Content, or OGC), not only of WotC's OGC but third-party's OGC as well.
I have read and reread the GSL, and it has NO MENTION of content sharing. Correct me if I'm wrong.
They say they're revising the GSL, hopefully for the better. The only way it can be better is to permit content sharing, at least among 3PP sources.