Excellent post! I'm glad someone finally pointed out the benefits of the smaller Gourgeist forms :) Your post was very well thought-out, but of curiosity, what base stats were you assuming for all of these calculations? We didn't have exact stats for the small form yet (but I'm updating the OP with the new info from serebii). Also, your Fire Blast calcs seem a bit high so you might not have factored in the power nerf down to 110.This thing makes a good spinblocker because the smaller sized one is one of the few spinblockers that can potentially beat most spinners slower than Tentacruel one-on-one thanks to unspinnable leech seed, Phantom Force to pass turns, and the movepool to 2HKO most common spinners/spikers.
Gourgeist-Small@ Leftovers
Ability: Frisk
EVs: 252 HP/208 SDef (or Def)/48 Spd
Calm/Bold
- Leech Seed
- Substitute
- Fire Blast
- Ghost/Grass STAB or WoW /Disable
Small sized ironically has the best bulk if it needs to reach the specific speed tier. Fully invested in HP give 314 HP for maximum mix bulk. 50 Spd EVs gives 246 Spd Gourgeist to outspeed and outstall 32 Spd Jirachi and essentially all spinners bar Excadril and Starmie. An -Atk reducing nature is used because it allows it to OHKO Forretress and 2HKO Skarmory 100% of the time after SR with Fire Blast. The rest of the EVs are put either into Def or SDef with a positive nature, giving 365/186 or 280/261 bulk which is still surprisingly bulky when faced with spinners. Alternatively those EVs can be put into SAtk for more damage on Fire Blast, though may not be worth the investment.
Aside from those bits, I was already going to add a set similar to this so great job on analyzing the matchups! Subseed small Gourgeist looks to be quite the pain to deal with.