Why it is everywhere
According to Pokémon Zone, Garchomp took 7.1% of the team slots at the Puebla Regional, tied with Rillaboom as the most used: about four in ten teams ran it. It won 50.5% of its games, normal for a Pokémon that often plays against itself.
Its Dragon and Ground typing explains a lot of that usage. Earthquake deals double damage to Incineroar and Raichu, two of the most used, and Electric attacks don't affect it. On top of that, it can bring three different forms in the same team slot, depending on the item it holds.

Without Mega Evolving: the fast attacker
Regular Garchomp has 130 Attack and 102 base Speed, and almost always runs Rough Skin: anyone who hits it with a contact move loses an eighth of their HP. Even Incineroar's Fake Out costs HP to whoever uses it.
In the Reg M-C online tournaments tracked by Pikalytics, its most common moves are Rock Slide, Stomping Tantrum, Dragon Claw, Earthquake and Scale Shot, with Life Orb, Choice Scarf or Sitrus Berry. Each one solves a problem. Earthquake, at 100 power, hits everyone around it, its partner included; Stomping Tantrum, at 75, hits only one target, never touches the ally, and doubles its power if its last move failed. Rock Slide hits both opponents and can make them flinch, and Scale Shot hits two to five times and raises its Speed at the cost of a Defense stage.
With Choice Scarf it doesn't need Tailwind to outspeed almost the whole format, but it is locked into one attack until it switches out.
| Form | Base Speed | Maximum |
|---|---|---|
| Garchomp | 102 | 169 |
| Garchomp with Choice Scarf | 102 | 253 |
| Mega Garchomp | 92 | 158 |
| Mega Garchomp Z | 151 | 223 |
| Sneasler, for reference | 120 | 189 |

Mega Garchomp: more power, less Speed
With Garchompite, Mega Garchomp keeps its Dragon and Ground typing and rises to 170 Attack, but drops to 92 base Speed. Its ability, Sand Force, makes its Rock, Ground and Steel attacks hit 30% harder in sand.
It is a Mega that needs a team built around it: with Tyranitar or Hippowdon setting sand, its Earthquake and Rock Slide hit very hard, and its lower Speed fits Trick Room better than a fast team. Remember that the Mega's Speed counts from the turn it Mega Evolves.
Mega Garchomp Z: a different Pokémon
Mega Garchomp Z, the new Mega in Reg M-C, changes almost everything. It loses the Ground type and becomes pure Dragon, with Levitate: Earthquake, including its partner's, doesn't affect it. Its Special Attack (141) is now higher than its Attack (130), and its base Speed rises to 151, one of the highest in the format.
That's why it plays as a special attacker, with moves like Draco Meteor or Earth Power, although Ground attacks no longer get its same-type bonus. Its type chart also improves defensively: as a pure Dragon type, it resists Water, Fire, Grass and Electric, and Ice no longer deals four times the damage. In exchange, it is no longer immune to Electric attacks: it only resists them.
| Damage taken | Garchomp and Mega Garchomp | Mega Garchomp Z |
|---|---|---|
| Four times | Ice | None |
| Double | Dragon and Fairy | Ice, Dragon and Fairy |
| Half | Fire, Poison and Rock | Fire, Water, Grass and Electric |
| None | Electric | Ground (from Levitate) |

How to beat it
First, check the opponent's sheet at team preview: Garchompite or Garchompite Z tells you which form is coming, and each is beaten differently.
Against regular Garchomp and Mega Garchomp, Ice is the most direct answer, since it deals four times the damage, and Incineroar's Intimidate lowers their Attack. Watch out for Rough Skin: contact attacks cost HP. Against Mega Garchomp Z, Intimidate no longer helps because it attacks with Special Attack, and Ground attacks don't touch it: Fairy or Dragon, which deal double, work better.
And in all three forms, don't hand it free turns: its Speed or its power punishes any setup turn. Our own reading: if team preview doesn't make clear which form it runs, prepare for the fastest one, because that's the one that wins the game before you can react.