Doing this will help you determine how drainage might work for your garden bed. Drainage is an essential part of any garden. What is actually under the gravel isn’t as important as knowing what that is.
Rock is hard for roots to penetrate. You will want to be sure that your raised beds are built high enough to allow your plants to fully root without hitting the hard gravel surface.
The question of whether raised beds need to be lined depends on a number of factors. Since we know you have a gravel base if you were considering lining your raised beds only to serve as a weed blocker, you may not need to.