External Gates

My players are saying that skinning high external walls with hammer works fine, but not the gates.

Players can say whatever they want.
Have you personally checked it out?

I think it's just a bit glitchy. I'm not able to skin walls or gates at all (I do have it enabled in my settings) but some others say they were suddenly able to skin the gates after not being able to before, some still not able to do it. Seems random. Not an issue that can be replicated consistently unfortunately.

Everything works properly and without glitches.
Most likely you and your players are standing close to the wall/gate, which does not allow you to change the skin, you need to move a little away from it.