I guess Dirt Mechanic has answered your question about gravel board, it is the treated board people put along the bottom of a fence to lift the actual fencing off the ground. Typically it is rough cut, about an inch thick and about eight inches wide, so reasonably heavy duty and would look reasonably natural. For stakes when I am doing jobs like that I usually use treated timber, the sort of thing used in roofing. It lasts a remarkably long time and provides a degree of soak away, and if you hit a rock and can't get it in any further it can be cut off short above ground. I would put the board in place with a stake each end uphill and then use it as a guide to hammer the stakes in down hill from it, just pressing them up against the board. It is the easy way to get it lined up, and the earth against it will hold the uphill side of the board pretty soon. You could clear some stuff that has fallen back behind the board to help it.
I have no idea of your climate and what might grow there, but there are some nice ornamental ivies, a yellow variegation would stand out. In the shade under the oak tree in my garden are a lot of naturalised cyclamen, nice flowers and an evergreen, variegated leaf with a good pattern. Some of the gooseberries (ribes) don't mind a bit of shade and might do well in front of the wall, giving you a crop as well as cover, but like I say I don't really know what would be suitable for your part of the world.