Zygarde outclasses Garchomp.
Not only does Zygarde have a better bulk than Garchomp, but, with its ability, Power Construct, it can use its full form. Also, in competitive, Zygarde is MORE useful than Garchomp. Despite Garchomp, having a mega form, its mega form sucks in competitive, and you can't really run anything for a Mega Garchomp set. Zygarde, on the other hand, gets access to MUCH more than Garchomp. You can run a bulky Zygarde to handle Fairy, Dragon, or Ice type Pokemon.
Also, Zygarde can OHKO a Garchomp using Outrage with a Life Orb.