We are talking here about holds. I don't think there is any rule that says you can't use a GPS for holds.
You can use what you like for en route ,as far as I am aware, as long as you remain at a safe altitude and carry the equipment mandated for the class of airspace you are in and class of flying you are carrying out.
There again if you are needing to do a hold you probably also need to have a working ADF so it may not help you...