Evil characters can worship neutral deities, and Gorum is pretty close to being evil. Irori is definitely neutral but is still an option.
The oni daimyo Akuma (LE) does martial arts and the malebranche Malacoda (LE) does martial skill - I suspect this is the closest to what you're after, and while simple its obedience is pretty good. General Susumu is a god of war and glory who happens to be LE. No obedience though. Similarly the Horseman of the Apocalypse Szuriel (NE) or the Queen of the Night Eiseth (LE) who do have obediences.
Asmodeus would be solid. The core tenet of Diabolism is that only the strongest, cleverest, and ruthless are fit to rule over others. If you get backstabbed or fail...you simply weren't the best.
Irori is definitely neutral but is still an option.
Yeah, the master of masters would be my choice. He is basically all about perfection, his sentinel boons are alright (IMO) and worshipping a deity of a different alignment can add a nice twist to a character. To back it up with lore:
So enlightened is Irori that even good and evil have ceased to have meaning for him. The Master of Masters does not appear to care to what ends his teachings are used, so long as the individual continues to strive for her own concept of perfection.
looking for perfection?
buff\healing, all weapons & armors, free item crafting and a nice kaboom if you need one?
half-elf(because elves loose con, and you might want bonus to str or wis) warpriest (calamity caller - elf only archtype) who worship one of the horsemen (deamon gods, i like Szuriel for war or Charon for death).
going into souldrinker (only need 2 levels for the pool and familiar but you can go deeper for early boons and some nice oblivion, depend on the horseman .normally youd loose some bab and warpriest abilities but since your using gestalt it might not be so bad).
net you:
calamity - 1d6 per 2 levels damage with tailored saves at will. enhanced into 1d6 per level a few times per day with added boom)
all weapons and armor proficiency (except tower and exotic).
warpriest's spells and abilities
a cacodaemon familiar for 1/day free soulstone from killed enemies (that there can get some big bucks depend on who you kill, see the soul trade). he can become invisible and shapeshift into a medium viper = invisible flanking buddy! (he doesn't need to attack, as long as he threat\can take aoo he can flank)
and a soul pool which can be filled with house centipedes souls (cost 1 cp each) using your own Enervation ability = free daily crafting (or at least 2X 1 cp turned into 500 gp soul point for crafting. and you craft about 1,000 gp per day so good enough,no?)
-note, you can also use Enervation on your familiar if you feel like it (he won't die, uses your hd after all), he (probably) won't mind. they are twisted littlie creatures...
My favorite is Demon Lord Haagenti. Easy obedience that helps prevent anyone else messing with your perfection. If willing to burn 3 feats (or 2 if your GM allows Diverse Obedience to work with Fiendish Obedience) Boons that can give you decent sla’s, proficiency in every weapon (and add features like brace, trip, and reach as a swift action) or a mutagen, and make you immortal.
If you do alchemist, of course, you'll access to the discoveries of vestigial arms and monstrous graft.
Haagenti would be my choice too. Perfection is subjective and fleshwarping and other surgeries has a real Repo vibe.