On the defensive set I would always stick to Leech Seed + Synthesis + Will-O-Wisp to have the best possible matchup against physical attackers. I prefer to use Foul Play to take on Lum SD / DD users like Lando-T, Dragonite and Gyarados a lot better and it also is the strongest move against pokemon like Torn-T, Latios and SD Gliscor after a boost.
Gourgeist-Super is already a bit gimmicky so you might want to stick with the standard defensive set or maybe go for Trick Room but I've played with one set I came up with a few days ago (not sure if it already existed in a lower tier)
Gourgeist-Super @ Choice Band
Ability: Frisk
EVs: 24 HP / 252 Atk / 232 Spe
Jolly Nature
- Trick
- Seed Bomb
- Shadow Sneak
- Rock Slide
The main reason to use this set is for the surprise factor. If your opponent finds out you're offensive, his counterplay against Gourgeist is a lot better and Gourgeist lost the biggest part behind its 'success' (surprise factor). Still, this set can cripple a lot of walls early in the game. Gourgeist lures in pokemon such as Ferrothorn, Clefable, Taunt Skarmory and Heatran, most of which become really useless after being tricked a choice item. Rock Slide lures in Flying- and Fire-Types that like to switch in against Gourgeist because of their supereffective STAB and a resistance to Gourgeist' Grass STAB. Between Trick and Rock Slide, there aren't many viable Fire- and Flying-Types that safely switch in against this set. Pair it up with something like Scizor or Volcarona and have fun after Heatran, Skarmory and Talonflame are gone.
Shadow Sneak is there for priority and although it is really weak against everything not weak to it, it still does its job at removing pokemon like Jirachi, Latios, and Alakazam. Seed Bomb is the strongest move for Gourgeist but because of bad coverage it is not really spammable unfortunately. It is a great way to weaken pokemon like Hoopa-Unbound and Mega Sableye, but other pokemon that aren't weak to it don't really care. The main issue with this set is that even with a Choice Band it still lacks power. Pokemon like Gliscor, Reuniclus, Breloom and Bisharp will always avoid the 2HKO from Gourgeist. An advantage however is that some of these switchins, like Breloom and Landorus-T, can't do much back to Gourgeist and will probably not switch in in the first place because of the possible defensive set.
My advice is to use this set early in the match to cripple annoying pokemon like Clefable or lure in pokemon like Talonflame. I have had a few matches where I could lure in both a defensive wall and a Fire/Flying-Type and if you succeed with that, Gourgeist did a great job. An example of this was crippling Clefable turn 1 with Trick and then weaken Kyurem-B with Rock Slide + Shadow Sneak (without Band it did a total of 55% (+10% LO recoil)) Making both Clefable as Kyurem-B pretty unreliable as Mega Manectric switchin (his ground type was Lando-T so Manectric swept in the end). Gourgeist is probably sacced in the first 10 turns more often than not but in many situations it does a pretty useful job in those first 10 turns. Don't be surprised if it fails a few times because this is just a gimmicky set for a gimmicky pokemon.
Edit:
So this set is pretty fun to use but I realised Trevenant can also run a similar set (not really a surprise I forgot Trevenant because why even consider it). It has a better ability, it hits a lot harder thanks to Wood Hammer and it has a much better matchup against Steel-Types thanks to Earthquake. I think if you want to use this set Band Trevenant is probably better how awful it sounds but you can still run this set because it doesn't die to +2 Lando-T / +2 Breloom / Mega Lopunny and you can lure in Flying-Types a lot better thanks to Rock Slide. Priority is also a nice tool Gourgeist has because of its low speed. You probably shouldn't use either of the two sets (Band Gour / Band Trev) but if you want to use some innovation in OU, both sets are definitely worth trying out.