for ex patriot QB's Cassel- before his one season starting for the pats, he was a CAREER backup- even in college. he was good enough to make the team and be the backup, but he hadn't started a game- until brady got hurt- since high school. while ultimately he was ok as a QB, he wasn't - except for 2 seasons - the pats season (where they were loaded and most likely WOULD have made the div championship/SB if brady was healthy) and his 2nd year with KC- a good quarterback- he was servicable.
Jimmy G. did start in college and -until he got hurt- granted it was a limited sample- he played at least as well as some of the better qb's in the league- was in he in brady/rogers/ben etc category? no, sample too small, but that sample showed - at least playing for the patriots- he was -apparently "Good".
will he be a great QB? only time will tell.