Garchomp is almost always a threat no matter what version of Pokémon you are playing. And in Pokémon Go, it is an absolute monster once you get into the later variants of the Go Battle League.
Like most Dragon-types in Pokémon Go, the matchup is very important when taking into account when and where you decide to use a Garchomp. The Dragon/Ground-type has a strong movepool, but will still struggle against many top picks in all but the Ultra League.
Overall, Garchomp is great, with 261 attack, 193 defense, 239 speed, and a Max CP of 4,479 at level 50. You can use it to hit like a truck in PvE, and it is an effective choice in high-level competition.
You will almost always be using the Dragon Tail/Outrage combo in both PvE and PvP movesets when you need Dragon-type output, purely because it has a max output of 50.2 DPS. Outside of that, Mud Shot should probably replace Dragon Tail as a Quick Move because it can easily build up energy for the costly Outrage and Earthquake combo.
Fire Blast is a fine tech, but outside of hitting Fairy-types, and being the only option Garchomp has to hit the heavily used Master League mainstay Togekiss, you should just forget it exists since the upside of Outrage/Earthquake, or even Sand Tomb is much better.
There are better alternatives to Garchomp in every meta, including multiple Legendary Pokémon or Pokémon that have a better typing for specific matchups. For that reason, you should probably not build your team around Garchomp in any league, but it is a fine addition to any squad in Ultra and Master Leagues.
If you don’t already have a Battle League-ready Garchomp, Pokémon Go’s June Community Day will focus on its pre-evolution Gible. You can likely expect Mega Garchomp to be dropping soon too, since Mega Altaria is being added for the Swablu Community Day on May 15.