My point with Toxic is that it's more flexible and easy to use, making it faster at the end of the day even if the damage takes a bit longer to ramp up within a single battle. Curse requires healing in some form. If they're relying on Hypnosis + Dream Eater, that's very slow to get going, rather than just Toxic -> switch to wall or keep attacking. If they're relying on items, that creates some tedium and the cost adds up in-game. If they're relying mostly on Pokemon Centers, that becomes very very slow grinding/progression.
Agreed that Gengar isn't particularly suited for stall tactics like Curse/Toxic, it's definitely built to be a sweeper, but that wasn't OP's question so I figured I'd humor them. Yes, the TMs can be tricky with Gengar, but ultimately it's not that great in-game without some significant TM investment.