Remove topsoil and soft subsoils
Road base thickness
The required thickness of road base, depends on the actual site subsoil conditions.
In general you will need to remove topsoil, peaty organic material, and softer subsoils. and replace these with compacted hardfill/roadbase.
If building your wall on bedrock, you may only require 25mm of roadbase, to level of the base
Most gabion walls do not require concrete foundations.