I ride my gravel bikes (geared and singlespeed) literally everywhere I used to ride my MTB's. This is SoCal mountains--steep, rooty, rocky, gnarly.
Granted, I was coming mostly from a rigid ss mountain bike so it wasn't that big a change--picking good lines, etc. Now, if you're a full sus mountain biker and used to just plowing over everything, the learning curve will be steeper.