How can you even begin to compare Garchomp to Milotic? What can Milotic do outside of supporting the team with Light Screen, inducing sleep and possibly scoring a surprise KO with Mirror Coat? Not much, really. However Garchomp is a whole different matter. Garchomp has an excellent combination of STAB moves along with defensive capabilities that rival Swampert's and a massive Attack stat combined with good Speed and Swords Dance and arguably one of the best traits in the game. Just pair it up with a Tyranitar or Hippowdon and your "counters" will suddenly only succeed at stopping Chomp 80% of the time. Throw in a Yache Berry and you'll need not one, but two fast Ice attackers to take it down, or would you perhaps prefer a Sand Veil + Brightpowder combo giving it a 28% evasion boost in the sand? After just one Swords Dance (which is extremely easy to set up with Garchomp's good defenses and excellent resistances) Garchomp can one-hit KO a large amount of OUs and the majority of those that aren't one-hit KO'd will die to the second hit and won't be able to OHKO it.
I have first-hand experience of how devastating Garchomp can be. I used to run a rather bad offensive team in Shoddy once and Garchomp alone has won me many matches with it. In most battles, Garchomp took down at least two Pokémon before going down. I ran a Jolly Life Orb Garchomp with Swords Dance and it's just ridiculous how many things get OHKOd after a Swords Dance. Gliscor was often the initial switch-in on my Garchomp and it quickly died to a Swords Dance Outrage. Outrage after just one Swords Dance (provided that Garchomp has Life Orb) is nearly a guaranteed 2-hit KO (49.11% - 57.99% damage) on a +Def 252 / 252 HP/Defense Bronzong. Swampert and Cresselia with the same EV spread take 97.77% - 115.10% and 73.42% - 86.26%. Vaporeon doesn't even stand a chance to survive the move. We're talking about some serious power here.
A well-played Garchomp will never go down without taking at least two other Pokémon with it. (or severely hurting them so that they are practically useless) There is no Pokémon that can switch in on Garchomp without risking to lose a huge chunk of its HP and Yache Berry and Sand Veil make any counters unreliable at best. I've rarely had trouble dealing with Garchomp, but that's only because most people don't know how to use it, and even in the hands of these clueless people, Chomp has usually taken at least one of my Pokémon down with it. However, after having used one myself, I became convinced that the thing belongs in Ubers. Even a complete moron can wreak havoc with Garchomp; now imagine that force in the hands of someone who can actually use it. Toxic Spikes is probably the best "counter" to Garchomp and that doesn't really say much considering Garchomp can come in and set up on any Toxic Spiker before they can get their Toxic Spikes on the field and even then, Spinners do exist. (which aren't nearly as common as they used to be, but then again, the use of Toxic Spikes has also been declining after the Tentacruel hype ended)
Also, keep in mind that just because there's a Garchomp-less test ladder doesn't mean it'll necessarily be banned. The people who have played on it will vote on whether or not they prefer the metagame as it is now or a Garchomp-less one. The countless Garchomp debates have accomplished nothing so I don't see why it'd be a bad idea to actually try a Garchomp-free metagame. How could we possibly know whether or not a metagame without Garchomp is less centralized unless we actually try it out?
