If the switch-in is a Pursuiter/Volt-turner, you can always Sub > Disable > switch
If the switch-in is something that is faster and can hit more than 75%, just run, because there isn't any that carries Pursuit.
Against an offensive team, you obviously do not come in on something that outright KO's Gengar. The fact that you are switching your Gengar to an offensive Pokemon either means you already took down the main wall to your sweep. If that's the case, you can always go for a Sub 1st, to scout for a switch.
If opponent switches, you Perish Song and repeat the ^
If opponent stays and uses something to break the Sub, just run away.