According to the Pokemon Global Link:
In Battle Spot Singles Garchomp mostly has a Yache Berry.
In Battle Spot Doubles Garchomp mostly has Groundium Z.
In VGC Garchomp mostly has an Assault Vest.
Overall Garchomp has mostly Focus Sash.
I recommend checking out the PGL as it can tell you Garchomps most popular item, most popular nature, most popular moves, which Pokemon Garchomp defeats, which Pokemon defeat Grachomp, and with that information build your Garchomp moveset and Garchomp team.
To find out more on Garchomp on the PGL, scroll down to Pokemon Ranking, click the "Choose a Pokemon" button, and search for Garchomp.
Also I think Dig can be replaced by Earthquake, Dragon Claw with Outrage, Iron Head with Stealth Rock/Roar(for singles), and return with Rock Tomb(for singles)/Rock Slide(for doubles).