So lucky you, you are on a Jumbo to SAN. I would say 70% chance B gate, B44 maybe. 20% chance of C gate. 5% chance A10, 5% chance A18 (in which case you really will have been extra lucky).

Assuming you get Galleries Club access, in your shoes, given the lack of better information I would first read up on the Passageway guide in the Dashboard, then head to the B gates for the really rather good lounge there. If I'm mistaken then it wouldn't be a big deal to walk to C or A.