It depends, really. There's quite a few poke in the teams that could potentially one-hit you as Gengar is primarily beneficial in SP ATK and SPD. If you have one that is going to prioritize SP ATK, you may go second - but at higher DPS. The alternative to your Gengar is having it hold a Quick Claw to balance it out. You could also use Black Glasses or any Ghost attack bonus item should you instead take the timid route.
As somebody who is in the market all day, everyday, I can tell you with assurance there are quite many Kanto Gengars with 20+ in both SPD AND SP ATK. So you could instead maybe purchase one of those, too. This also all goes without saying fully EV trained Pokemon will maximize your potential to win, as well.