You can re-EV train Pokemon, btw.
It's better to put all of a Modest Goodra's EVs in to Sp.Attack and HP. This maximizes its ability to sponge special hits (forget about it tanking physical attacks). Give it an Assault Vest and replace Infestation with Sludge Bomb or Focus Blast.
Gengar's moveset isn't bad, but these would be more effective:
-Shadow Ball, Sludge Bomb, Focus Blast, Destiny Bond
-Shadow Ball, Sludge Bomb/Focus Blast, Substitute, Disable
Tyranitar's a Pokemon that has several different viable sets.
If you don't wanna re-EV train it, it MIGHT be possible for it to re-learn Dragon Dance. But I'm not sure about that, I have yet to confirm it. And since I'm too lazy to type out all of Tyranitar's sets (since almost any one of them could work for your team), here's the links to the pages with the sets:
http://www.smogon.com/bw/pokemon/tyranitar
http://www.smogon.com/forums/threads/tyranitar.3490010/
Zoroark's not the best Dark type, but you can use it if you want. If you wanna use a Nasty Plot set, it needs a Focus Sash. Zoroark is a very frail Pokemon.
As for your remaining 2 slots, something that can take physical attacks, particularly Fighting type ones, would be good, and something to deal with Fairies, who threaten 3 of your Pokemon (Gengar can't handle everything). Steels handle the latter well. Try Ferrothorn or Scizor:
-Ferrothorn w/Leftovers
Nature: Relaxed/Sassy
Ability: Iron Barbs
EVs: 252 HP, 40 Defense, 218 Sp.Defense OR 252 HP, 126 Defense, 126 Sp.Defense
Moves: Leech Seed, Stealth Rock, Gyro Ball/Thunder Wave, Power Whip/Protect
Do not use Gyro Ball and Thunder Wave together.
-Scizor w/Choice Band
Nature: Adamant
Ability: Technician
EVs: 248 HP, 252 Attack, 8 Sp.Defense
Moves: Bullet Punch, U-Turn, Pursuit, Superpower/Brick Break
or
-Scizor w/Life Orb
Nature: Adamant
Ability: Technician
EVs: 252 Attack, 252 Speed, 4 HP
Moves: Bullet Punch, Swords Dance, Brick Break, Bug Bite/X-Scissor
or
-Scizor w/Leftovers
Nature: Adamant
Ability: Tech
EVs: 252 HP, 40 Attack, 216 Sp.Defense
Moves: Swords Dance, Bullet Punch, Roost, Bug Bite/U-Turn/X-Scissor
As for a Fighting type/physical wall, Gliscor and Gourgeist do the job well:
-Gliscor w/Toxic Orb
Nature: Impish
Ability: Poison Heal
EVs: 252 HP, 244 Defense, 12 Speed
Moves: Eathquake, Knock Off, Roost, Toxic
or
Gliscor w/Toxic Orb
Nature: Impish
Ability: Poison Heal
EVs: 244 HP, 40 Defense, 224 Speed
Moves: Substitute, Toxic, Earthquake, Protect/Roost
-Gourgeist (Super Size) w/Leftovers
Nature: Impish
Ability: Frisk
EVs: 252 HP, 252 Defense, 4 Sp.Defense
Moves: Leech Seed, Seed Bomb, Shadow Sneak, Will-o-Wisp