There's a reason why Flame Charge isn't used competitively. I've already addressed this twice now:
It would take 2 wasted turns to get Super Size up to Small form's speed, no opponent is just going to sit there and let you boost for free.
As much as I love our new Pumpkin friends, if you think Trevenant has a horrible movepool you're completely mistaken. Gourgeist is the one with the horrible movepool: Trevenant has Earthquake, Shadow Claw, Horn Leech, Wood Hammer, X-Scissor, and even Poison Jab, and Gourgeist doesn't get any of those moves. If you want a Pokemon with better attacking options and more reliable recovery, Trevenant's the way to go.
However, Gourgeist does have a ton of versatilty, much better physical defenses, and Shadow Sneak for priority. Small forms are much better at utilizing WoW and Leech Seeding. It also packs a potent TR set with Explosion. You should try both and see which strengths are most important for your team!
Edit: myself and others have been testing Egg moves in the research thread, and sadly the most notable moves can't be passed to Gourgeist.
But i had stated above im using the small size Gourgeist obviosly a super size has better moves to work. The smaller one coming off higher base speed could use it its most likely not optimal and pardon me for not flipping back through all th epages to find the moveset that was suggested for a "SMALL" Gourgiest im sorry if i come off as rude but your reply felt so to me ill have to look back and see the seeding and WoW set though i find those interesting