Good stuff, good stuff.
-The most common sleep-inducing moves are still really rare. And they're Sleep Powder/Spore, which Gourgeist is immune to cause it's a Grass type. Make it either Pickup to find some neat junk or Frisk to scout ahead for surprises.
-Aggron is really not that good. It's got a lot of physical bulk, but you've got a Gourgeist! You don't need bulk, you need a good attacker and Aggron...isn't one.
If you REALLY wanna keep it, ditch Rock Polish. Aggon can outspeed stuff at +2 if you have near-maxed speed, but even then, it just doesn't have the power to be a Rock Polish sweeper. Also, Iron Head or Heavy Slam are way better as your Steel STAB because...they can actually hit things. To replace RP, your options are limited if you're keeping EQ...Brick Break? Or use Munchlax to get Body Slam for the paras (you can keep EQ since Munchlax gets it too).
Buuuuuuuuut, I'd really advise replacing it. The first thing that comes to mind is Rhyperior because it's also a slow physically bulky (maybe even bulkier because of Solid Rock, depends on how you look at it) Rock type, but it's got a really good attack stat and great movepool (Poison Jab, Megahorn, all elemental fangs, Crunch/Shadow Claw, Metal Burst/Counter, Brick Break/Hammer Arm). You also have other Rocks like Archeops, any Shell Smasher, Gigalith, Tyrantrum, Aurorus, Relicanth.
For Steels, Durant is godly. Hustle gives it effectively 190 base Attack, and it gets Hone Claws to boost that even more and make up for Hustle's accuracy drops. Other great Steel types are Metagross, Bisharp, Empoleon, Excadrill, Escavalier.
If any of those^^ interest you, say so and I'll come up with some sets.
-Crobat is way better as a Nasty Plot attacker. Air Slash, Venoshock, Nasty Plot, Hidden Power Fighting/Ground. It's especially good if you have some Toxic Spiker around, even if it's just a level 1 Sturdy Pineco.
Though it might end up being useless if it can't set up (and physical Crobat is not the best), so if you want some replacements:
For Flying types, there's Honchkrow, Archeops, Swellow (if you get the Toxic Orb), Staraptor, Simple Swoobat, Sigilyph, Braviary, Vivillon and Hawlucha.
For Poison types, you've got Muk, Weezing (though it's only really usable as physical wall. which it does great, but not as well as Gourgeist. though it does get Destiny Bond), Nidoking, Toxicroak, Dragalge, Venomoth, Tentacruel, Scolipede and Roserade.
Again, if curious, I can explain and give sets for em.
-Thing is, we don't have Thunder. It's unknown if we ever WILL have that. Instead, since I see you've got that Ninetales, get Heliolisk. With Solar Power, it's gonna be hitting insanely hard. Also has neat coverage options like Surf (which you shouldn't use in sun, but still), Grass Knot and Dark Pulse. GK, Thunderbolt, HP Ice and DP are it's best set IMO. At least until we get a Hyper Voice tutor, which we may never get because HP is really good for Sylveon/Aurorus.
-Again, Solar Beam is like Thunder. Breed Energy Ball for now. For Heliolisk, you'll obviously want to get it a Heat Rock to extent Drought duration. Also, Will-o is useless. Ninetales is best of either setting up and sweeping or coming in, nuking something with Drought Fire STABs and leaving. I've found the sweeping option to work decently well and Fire/Grass/Rock is perfect coverage (Ground is pretty good too), so get Hidden Power Rock/Ground. And then, either try being a sweeper and keep Nasty Plot or go with hit-and-run and replace NP with Extrasensory (of course, you don't have to NP every single time, you can still hit'n'run. and Psychic coverage is nothing special, so yeah)
---Now I COULD give some recommendations for your 6th team member, but that'd be way easier to do if I knew your first 5 for sure. To rephrase, you keeping Aggron/Crobat or what are you replacing them with (or do you need me to go into detail about their possible replacements)?