After backtracking all the way back to Gamboge City, I took another route and reached Nephrite City, a place full of Pokémon Rangers.
It has a Safari and a Normal type Gym which is blocked until you obtain a package from the Safari.
>one Safari Catching session later.
The gym is now open and I must say that I'm underleveled for this (the gym's ace mon is like level 35 while my best mon is level 30)