International has A and G gates opposite of each other, but they are not currently connected airside. You can only go C-D-E-F-G and A-B airside. (Next year when construction finishes you will be able to stay airside for everything but from A-G and G-A will require walking the length of the airport.).

If you're originating in SFO and want to access AMEX lounge then clear at G, walk to T3F.. AMEX is right there. Then walk back and exit at G, walk across the international terminal to the other side and clear at A. If you're arriving on DL at C, then you can walk to AMEX lounge airside, and then exit at G to go to A.