Okay, this team is pretty flawed. I'll try my best.
Garchomp
Don't use mega garchomp. Normal garchomp is much better.
Garchomp @ Life Orb/Choice Scarf
Ability: Rough Skin
EVs: 4 HP / 252 Atk / 252 Spe
Jolly/Adamant Nature
-Swords Dance/Outrage
-Earthquake
-Fire Fang
-Whatever coverage you really need
Use this instead. LO garchomp has about as much power as mega garchomp. Its speed is much better. If you want to run a standard garchomp, go with a scarfed set.
Metagross
Normal metagross just isn't very good. Use mega instead.
Metagross-Mega @ Metagrossite
Ability: Tough Claws
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
-Meteor Mash
-Zen Headbutt
-Earthquake
-Ice Punch
Pretty self explanitory.
Tyranitar
Perfect, except ice punch > beam, and you're missing EVs. 248 HP / 252 Atk / 8 SpD should work well.
Blaziken
Blaziken is banned in OU, but looking at your moveset, scolipede could do the job well.
Scolipede @ Black Sludge
Ability: Speed Boost
EVs: 228 HP / 252 Atk / 24 Spe
Adamant/Jolly Nature
-Protect
-Baton Pass
-X-Scissor
-Poison Jab
252+ Atk Choice Band Talonflame Brave Bird vs. 228 HP / 0 Def Scolipede: 140-168 (44 - 52.8%) -- guaranteed 3HKO after Black Sludge recovery
(140, 144, 144, 146, 146, 150, 150, 152, 156, 156, 158, 158, 162, 162, 164, 168)
I never realized how bulky this thing is until now. Essentially the same moveset as blaziken, but with scolipede's STABs.
Gengar
...No. If you want something like this, try bronzong.
Bronzong @ Leftovers/Big Root
Ability: Levitate
EVs: 252 HP / 132 Def / 124 SpD
Bold Nature
-Block
-Hypnosis
-Dream Eater
-Flash Cannon
Much better. Bronzong is SUPER tanky, with 116 in both defenses. Unlike with gengar, the opponent can't simply switch out to nullify the effects of sleep, aka dream eater functioning. Plus, this idea works much better on a tanky pokemon, due to the benefits of gaining HP.
Gyarados
Perfect, except choice specs? XD try life orb.
With a couple minor tweaks, this team is almost perfect... WAIT. Other than bronzong, you have no special attackers! However, I have a solution to pesky sableyes and rotom-washes crippling your team with will-o-wisp, or landorus-therians intimidating everyone on your team!
Chandelure @ Choice Scarf
Ability:Flash Fire
EVs: 44 HP / 252 SpA / 212 Spe
Modest Nature
-Trick
-Fire Blast
-Shadow Ball
-Energy Ball
Use this, and all your main previous threats go out the window! And who to replace?... Well, that's up to you. Have fun battling!
