Gandhi was the oldest WSSM but none knows this information. Perhaps he beat Ryuma even, or has a feat not even Mihawk himself can pull under his belt.
In this case, I could see Mihawk being taken down. Actually, possibilities are high. But I don't know by who. Nearly everybody is going down before Gorosei imu fight and as u say I dont see how Zoro can fight Mihawk and then go after supposedly less strong opponents -ghandy.