Gravel is a somewhat worthless raw material. It is in fact part of the crafting recipe for Reactor Components, but the amount needed for those is so small compared to the sheer amount of Stone/Gravel a mature station will have lying around that it hardly matters. It does have limited application as a sabotage tool, filling up inventory space in a enemy Station, but this is more trolling than an actually effective tactic.
Gravel may be a part of concrete if it is ever implemented (see here), but this not yet confirmed.
These are assembler recipes that use Gravel: