A gravel bike can indeed be used as a road bike. The primary distinctions are tire choice and gearing. By switching to slick road tires, rolling resistance and aerodynamics will be drastically improved making the gravel bike act more like a road bike. Another option is to set up to not as wide as gravel cassette, but not as narrow as road cassettes in order to find a gear that is best suitable for the cadence on the roads. Having glue on road wheels is a convenient way to switch between road and gravel riding.