Nope, setting the Unit Version Date didn't help for me.
I did finally get a Gato as the default by changing the Mission year from 1943 to 8/12/1941. I looked at another mission that used a Gato and set the Mission and Sub to that date. It worked.
I'm guessing that the game was saying that the Gato was out of commision and so it gave me the next sub, a Gar?
So, I need to know the Entry and Exit date for 'every' sub to set the Mission dates if I want to pic a specific sub??