I don't recall hearing or reading anything about Retros except in Privateer and Righteous Fire. Is it just a local cult, or does it have a further reach? Is there any evidence either way?
It is *not* exclusive to Gemini, though of course it is (secretly) based there. The 'Interview with a Frontiersman' story has characters on Sheol referring to Gemini as being the far-off frontier... and it ends with Retros capturing their base.
Only in that they're both 'crazy' religions in the Wing Commander universe. Theologically speaking they're complete opposites - the Church of Man puts humanity above everything else while the McDanielites (Pilgrims) vehemently demonize it.