From a coverage-based point-of-view, I'd give it Iron Head as a move tutor move, Dragon Claw and Earthquake as TMs, and something filler (maybe Sandstorm because of its Sand Veil ability).
Iron Head would cover for Garchomp's weaknesses to Ice and Fairy.
Dragon Claw would cover for its weakness to itself as well as provide STAB.
Earthquake would just provide STAB.
Sandstorm would aid in its Sand Veil ability.