Not sure of the exact details like seejaie appears to be but I can corroborate that after a fairly recent steam update I am no longer able to use my macbook air to make Civ V turns through GMR. When I've tried I get the same error on upload as seejaie. Fortunately, I have access to a Windows machine where I verified that I could make the turn and upload it without issue so this is strictly a OSX issue.