Sure you can. The regular nail polish of course can be applied over a gel nail polish. The gel polish underneath would be dry and act as a base coat for the regular nail polish. The only problem is that the regular polish would work nicely if it has good quality and opacity but a cheaper polish might not show up too well over a gel polish.
Moreover, if you use the gel polish as the base coat, you need to take the color of gel polish into consideration. Generally, a darker colored regular polish can work well over a lighter gel nail polish base and a lighter colored regular polish is not visible over a darker gel polish coat.