I've been looking in the secret entrance to the wayward cave north of oreburgh, where you have to move a rock and then go down some stairs. I've been here for over 45 minutes and still a Gible hasn't appeared, am I doing something wrong?

Theyre uncommon. Takes some time, theyre down there.
Serebii has more detailed information for you.

Thanks for the awnser, I ended finding him and he's now at level 46, getting ready to get him to evolve into Garchomp and face the Elite 4!