As much as I hate to say it, you're going to have to do some re-breeding on some of your Pokemon, particularly Garchomp. Under normal circumstances, its nature should be Jolly. A Careful nature isn't good on it because it's more of a "defensive" nature, and Garchomp is more tailored towards sweeping than defending/supporting. When Black 2/White 2 come, you can teach Garchomp Outrage; this should go over Fire Blast.
Excadrill appreciates the freedom of switching moves to hit opponents hard with the appropriate move, considering that Choice Band leaves it locked into a single move as long as it's in battle. Use and Air Balloon as the item for a temporary immunity to Earthquake. Also, use an Adamant nature; Bashful does absolutely nothing on a Pokemon's stats.
Gigalith is terrible in OU because of its somewhat widespread weaknesses and shallow move pool. Tyranitar should be in there instead because it sets up Sandstorm and has a much better move pool:
-Stealth Rock
-Crunch
-Fire Blast
-Ice Beam/Superpower/Stone Edge
Nature: Sassy/Hasty
EVs: 252 HP/64 SAtk/192 SDef (Sassy) or 56 Atk/252 SAtk/200 Spe (Tyranitar)
Item: Leftovers
Ability: Sand Stream
What exactly is Ludicolo doing on the team? Virizion is a better alternative because of its ability to help with bulky Waters:
-Calm Mind
-Giga Drain
-Focus Blast
-Hidden Power (Ice)
Nature: Timid
EVs: 4 HP/252 SAtk/252 Spe
Item: Leftovers/Life Orb
or
-Swords Dance
-Leaf Blade
-Close Combat
-Stone Edge/Hidden Power (Ice)
Nature: Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Leftovers/Life Orb
or
-Work Up
-Close Combat
-Giga Drain
-Stone Edge/Hidden Power (Ice)
Nature: Hasty/Naive
EVs: 104 Atk/152 SAtk/252 Spe
Item: Leftovers/Life Orb
or
-Reflect
-Light Screen
-Taunt
-Giga Drain/Close Combat
Nature: Timid/Jolly
EVs: 252 HP/4 Def/252 Spe
Item: Light Clay
Gallade should use an Adamant or Jolly nature to take advantage of its high Attack or decent Speed. Replace Leaf Blade with Night Slash or Trick (latter comes as a B2/W2 move tutor). Give it Expert Belt or Life Orb
On Forretress, just so you know, Gyro Ball gets stronger the slower you are than the opponent and adding in Speed EVs makes it weaker. Using a 0 Speed IV (or something close to it) with no Speed EVs makes Gyro Ball very strong. Make the EV spread 252 HP/176 Def/80 SDef for more bulk. Explosion got nerfed in the generation shift (it no longer halves Defense before it hits). Rapid Spin is the primary option in the last slot to get rid of opposing entry hazards.