Amateur Garruk historian / Garruk superfan here...
Garruk's last known whereabouts was on Shandalar. Jace put a hedron in his head to reverse/stop the progression of the veil curse. When this was done to Ob Nixilis he lost his wings, and potentially his ability to planeswalk ( tho it is debated that zendikar, the plane, took his spark from him in his weakened state, due to the planes "zany" leylines of Mana).
So it could very well be that right now he is stuck on Shandalar...but I doubt this.
The cool part is that people on Shandalar are very wary of walkers (Google Arzakon/ Shandalar game)...and he's now a walker killer...so there could be some sort of story to be told where Garruk is killing walkers on Shandalar, pissing off walkers in the multiverse while Shandalarians are praising him as some sort of saviour/defender.
Or he's just secretly/ quietly going from town to town killing walkers...quietly enough that no one knows.