That has nothing to do with gravel
I'd say you're missing a seal or spacer. The Trek BB90 bottom bracket comes with a plastic tube. Maybe that's not on there.
https://media.alltricks.com/hd/58fa09b19a475.jpg
Also the crank may not have been tightened in the right order. The 'barrel' bolt first, then then pinch bolts -
https://www.parktool.com/blog/repair...ession-slotted . If you don't do this, the crank will move around.
The frame may also be out of spec.