My Gourgeist knows it and I was wondering if I switch it with something else or not.
I have found Trick-or-Treat to be very useful, due to making Pokemon weak to your Phantom Force who would otherwise have immunities. It also can shut down Dark types due to the added weakness and gives Gourgest great coverage.
Competitvely, I wouldn't recommend it, due to Pokemon already dropping you to low HP the turn you use the move.
I have found it a very useful move in competition myself but my it fits the role I have my Gourgiest fill of a stall/set up unit for other Pokemons move sets. Substitute followed up by trick or treat, then sitting behind the Substitute using leech seed and foul play with the giest works very well for me though mine is built as a HP/defense tank to support rest of line up.
It makes everything weak to my dark types with their STAB dark moves. I don;t run a STAB move on my Giest as it has such low Attack and Special attack foul play hits harder still being based on targets attack.