When putting gravel under a shed for a foundation the depth of the gravel for the shed foundation should be at least 4 inches to 6 inches.
You can go a bit deeper but there's not much need too for a smaller shed.
You can also build a shed on skids and not need a foundation either.
You can build a shed without a foundation.
If it's a small shed you can just build the shed on skids which would make the shed a portable shed.
When you build the shed as a portable shed you make it so that it becomes personal property and not real estate property.
Your taxes are usually lower on a portable shed than a permanent shed.
Also you don't need permits to build a portable shed whereas sometimes you need a permit to build a shed with a foundation.
Another benefit of the portable shed on skids is that you can easily move the shed to a new location or hire someone to move it for you.