John Dall is also first rate in the Bette Davis film The Corn is Green. Yes, pity his film legacy is 'light'. That happened to many young actors that got there start in the late 40s because after their initial studio contract ended in the 50s, the studio system was starting to break down (T.V. being a reason), and studios were a lot more reluctant to sign actors to long term contracts like they did in earlier decades.
Gun Crazy is terrific, but note that Peggy Cummings career faded even more so than Dall's.
Oh yes, I forgot about The Corn is Green! Great film with a terrific cast/acting.
I heard that John Dall didn't have a better career because he was homosexual, but there might have been other reasons, like the one you mentioned.