When I had a gravelled area done the ground was levelled & firmed & a membrane put under the gravel. It's been fine & is walked on occasionally. You'll find the membrane at builder's yards, (although they'll try & sell you the very expensive stuff), diy & garden centres & it can usually be bought off a roll by the metre or in pre-cut packs.
(Plastic which has been pierced to allow rain through isn't a good idea as the gravel slides about on it if you do need to walk on it).