Yes. The visible gravel board sits beneath the fence panel and adds its own height to the finished boundary, so it must be included when choosing the panel and post length.
Yes. A gravel board forms part of the visible fence height because it sits beneath the timber panel rather than within the panel height.
For example, our 6 Inch Smooth Concrete Gravel Board is 150mm high. Placing a 1650mm closeboard panel above it creates 1800mm of combined visible panel-and-gravel-board height.
A 12 Inch Smooth Concrete Gravel Board is 305mm high, so the same fence panel would finish 155mm higher than it would above the 150mm board.
The same principle applies to a 150mm wooden gravel board. The gravel board height is a separate component and should be added to the panel height when planning the complete boundary.
This also affects the post length required. The post has to accommodate the visible panel and gravel board as well as the section used below ground. Our fence post length calculator includes gravel board height as part of that planning calculation.