Haven't been into Game Maker for years, but I know their language and C# as well and my 2 cents are:
GML is a very easy, flexible and simple to use scripting language but it's not scalable , even if you go C# if the core engine can't handle advanced stuffs such as bindings and reflection, then it's like six of one, half a dozen of the other, no gain.
Unity is much more mature on C# than Game Maker and yet they don't support everything, so if you want to go that route I at least recommend you switch to Unity.