Correct me if I am wrong, but I believe CX flights are only at concourse A and are only confined to that concourse. So even if gates are available at concourse G, they won't be relegated to that section. At SFO, I've never seen airlines assigned at concourse A (i.e. CX, BA) have their a/c's parked at the G concourse or airlines assigned at concourse G (i.e. SQ) have their a/c's parked at concourse A.