I think Gordon is quite likely gone, probably as a DFA, before next season starts.
As for Farmer, I feel less certain about, for 2 reasons. First, he has a better history in this league than does Gordon. Second, he can also fill in as an emergency catcher, even though he hasn't had to this season.
I don't see a huge problem with having both Castro and Farmer on the roster, provided nothing radical or unexpected happens in the meantime.