I doubt such a Gible would turn into a competitively viable Garchomp, but the Battle Tree is not the same. I believe you could make it work, but don't take my word for it.

As for the best way to EV train, that would be SOS chaining. Here are the best SOS chaining spots for EV training.

An alternative is Poke Pelago's Isle Evelup.

On that island, you can put groups of 6 Pokemon into the island to play, and then give them specific drinks. These drinks will improve their stats. The Pokemon are put into the island for play sessions that last 30 minutes, and you can set how many play sessions the Pokemon go through.

Depending on the level of the island, the Pokemon gets 1, 2 or 4 EVs respectively every play session, so, at Level 3, they gain 4 EVs every 30 minutes, meaning to maximise a stat (i.e. gain 252 EVs) you'd need to have 63 sessions lasting 31 and a half hours. Poke Beans will speed this up so that the play sessions only last 15 minutes per session, making the time it takes to max a stat out 15 hours and three quarters.

Although less efficient than SOS chaining, the process is simple, passive, and your 3DS can be turned off without stopping the timer.