I have extensively tested the following Gourgeist-S set, and find it to be quite good:
Gourgeist-Small (F) @ Leftovers
Ability: Frisk
EVs: 252 HP / 72 Def / 184 Spd
Timid Nature
IVs: 0 Atk
- Leech Seed
- Substitute
- Protect
- Will-O-Wisp
One of the main things that Gourgeist-S has over its huge form is a FAST Will-O-Wisp. I can't count how many times I've burned things like Mamoswine, CB Dragonite, Adamant Mega Kangaskhan, and Excadrill before they had a chance to attack. Furthermore, Leech Seed + Substitute + Protect makes Gourgeist a great annoying Pokemon, and simultaneously gives him a reliable source of recovery.
I tried testing an attack on Gourgeist-S (by dropping Protect for Phantom Force) for a little while, and found it to be a bit of a waste of time, since he rarely uses it, and it doesn't do much damage anyway. Doing this does make him completely shut down by Taunt and Grass-types with Natural Cure, though, but giving him the ability to both scout and SubSeed in exchange is entirely worth it.
I like the idea of a fast Gourgeist, but the problem is that the surprise factor isn't there, because the size clearly displays how fast it's gonna be. However, I see the potential, and definitely the fun in being able to threaten stuff like Mamo and D-nite. A question I have is do you have "Spd" mean speed or special defense? If it's speed I get why, but I would like to mention that you only need 12EVs in speed to outspeed Mamo at all and 104EVs in speed to outspeed Jolly. A cool thing about 104 EVs in speed is that you go from 284 speed to 286 for just the 4 extra over 100EVs in speed, so two for four, but you lose out on 4 defense this way. All this is done at Lv100, not 50, so the numbers could be off for what you're calculating around, I just prefer Lv100(and assume most people do) because it's a lot cleaner number-wise. I'm assuming that 0IVs in Atk is for Foul Play as well.
At any rate:
Lv50 Gourgeist-S@Leftovers - Bold/252HP/44Def/212Spe
-Leech Seed
-Substitute
-Protect
-Will-o-wisp
This optimizes the numbers, giving you 2 more points in defense at Lv50 than a Timid nature does with a spread of 252HP/144Def/112Spe, while yielding the same speed(146 to outspeed Jolly Mamo/D-nite). Below is the same set optimized for Lv100.
Lv100 Gourgeist-S@Leftovers - Bold/252HP/52Def/204Spe
-Leech Seed
-Substitute
-Protect
-Will-o-wisp
Obviously you're going to want to use the first spread if you're gonna be playing a lot of auto-50s.
Also, for a Pain Split set, I'd say try as close to 0HP EVs on a Gourgeist-S as possible, running the same speed EVs as the above sets(to still outspeed base 80s) and then pump the rest into the defenses, as much as desired. Something like this:
Gourgeist-S@Leftovers - Impish/20HP/44Def/232SpD/212Spe
-Pain Split
-Will-o-wisp
-Rock Slide/Shadow Sneak/Destiny Bond
-Seed Bomb/Shadow Sneak/Phantom Force
The 20HP EVs ensure that you heal for 16hp on leftovers, which is just a numbers thing, but then the rest can be placed into SpD. Alternatively you could use the EVs to max out Defense and then put the remaining in SpD, it's all up to what you need this thing to do I guess. A hilarious thing you can do with Gourgeist-S is run Destiny Bond(breeds onto it), probably catching a lot of people off guard with a fast wall that can D-bond. I should also note that with an Impish nature you get a free point of defense when you put 240 EVs in, jumping from 372->374(according to Showdown), which is pretty cool for you min-maxers.