To interoperate with GDScript from C#, you have to use Godot's generic functions from
Godot.Object (a bit like reflection) because there are no actual C# functions and variables you can use.
So to call a function, it would be like this:
Node node = GetNode("Node/With/Your/Script/On/It")
node.Call("function_name", argument1, argument2);
Getting and setting variables:
Vector3 v = (Vector3)node.Get("variable_name");