Tbh, I would actually go for Gyro Ball here. Reason being majority of the VGC metagame is pretty heavy, so Heavy Slam is somewhat hampered. On the contrary, quite a bit of the VGC metagame has 70+ speed (save the Trick Room users) so Gyro Ball hurts like a giant steel-clad serpent. PP isn't usually an issue because most VGC games last no more than 15 turns, and 8 PP is more than enough for one game.