Hope that you already played through the game, because I am giving spoilers.
In my opinion Garchomp is a good physical attacker and it's a pseudo legendary, but if you don't like it's base 95 defense, swap it out for a bulkier Pokemon like Umbreon.
Garchomp can get moves like Dragon Claw as a good STAB Dragon move, and Earthquake as a good STAB Ground move. It also gets good coverage moves like Stone Edge and Iron Tail.
You aren't going to get Garchomp though until at least the 8th gym, where it's going to be actually useful with Earthquake. (if you have the TM)
Earthquake is Super Effective against Jolteon, Raichu, Luxray, and Electivire and can one shot basically everything on Volkner's Team.
Moving on to the elite four...
Garchomp can't do much to Aaron unless your running Stone Edge or Fire Fang, so when Drapion is sent out, hit hard with a Ground move.
Garchomp is super effective against Golem and Rhyperior with a STAB ground move.
All of Flint's Pokemon are weak to STAB ground moves with Garchomp, but watch out for Hyper Beam Magmortar. (actually, that's not really a threat)
Lucian's Bronzong has Levitate, so you aren't going to be hitting it with Garchomp unless you put Fire Fang on Garchomp. It can still do good in this battle, though.
Cynthia's Lucario is weak to a STAB ground move, so try to get Earthquake on Garchomp to guarentee an OHKO.
All in all, if you are willing to train up a Gible/Gabite, you should use it ingame.
Hope this helped! :)