There are two things that will bear on this. Title 24 Requirements and the building code requirements.
In regards to Title 24 energy code most homes are required to have either certain amount of roof insulation or a cool roof. I’m not sure if there is a cool tar and gravel roof system at this time. Otherwise you would need to use an special gravel or a coating to obtain this rating.
Second is weight or layers. If you already have two layers of roofing then they must be removed before a new one is installed. This is a code requirement.
The real question is why would anyone want a tar and gravel roof anymore when there are so many better alternatives.