I ride my gravel bike on MTB trails on a regular basis. It makes the same old trails that I've been riding for years into a new and challenging experience again. You can't just bomb down descents, so you have to be more aware of your line choice, but my gravel bike is actually faster up most of the climbs than my MTB.
A lot of the gravel events in my area include singletrack sections to link up different gravel roads. Some of them are pretty rocky/rooty trails and it quickly becomes obvious which riders are comfortable with riding their gravel bikes in those conditions.