I have gravel paths, pea sized on stairs and chippings on a path. You need to rake them back up the slope occasionally to keep an even depth though. I would be tempted to get a plank of wood and wedge it in place to keep the gravel in place for what you want to do though, the wood need only be a few inches tall.

Thanks for your advice!

I have 1/2" gravel 4" deep on a weed control membrane which is not slippery, on a slight slope and it never moves, unless I walk on it. Depending on traffic? I agree with above I rake them over once a year and it looks good, the reason for the larger size gravel is it is too big for cats to use.

Thanks everyone for the tips. I am hoping for some good weather this weekend so I can finish my daughters garden before heading off home to Germany to do my own garden that is now in need of some serious care!