The under gravel filter in actually a plenum filtration(without the tubing, a very old filtration system which works similarly to a DSB...
The top gravel will be exposed to oxygen rich water which aerobic nitrifying bactera will converts the generated ammonia into nitrites and the bottom which is devoid of oxygen will inhibit anaerobic bacteria, which converts nitrates into nitrogen.
You can continue using it, and add more sand onto the top... Depends on how much gravel you have in your tank right now...