What level would a Hunter evolve into Gengar?
For that matter, what levels would any trade evos evolve here?
Or would it be to do with happiness?
It's just that I can't find any information on this particular problem, besides:

You go on to talk about item trade evos, but there is nothing on those who do not trade evolve with an item.

I'm pretty sure you just post in the evolution forum to have it evolved whenever you want.

For those that normally evolve by trading without an item, in this RPG you to use a certain item on them as if it were an evolution stone. I see I forgot to add Haunter to the list as well as the others from 1-150. I need to update that post a bit. Pokemon that evolve this way can evolve at any level as long as you have the required item. Just post in the evolution forum when you want to evolve it.