I realize I'm late here but for anyone else's reference:
Your GDNative class should still have the function get_parent() if it inherits from the godot::Node class.
So let's say you wished to perform the following (GDScript version):
var parent : Node = get_parent()
print(parent.name)
The GDNative equivalent would be:
godot::Node* parent = get_parent();
godot::Godot::print(parent -> get_name());
When I'm a little lost I find it useful to check the header files for GDNative. In the case of Node.hpp it would be in godot_cpp/include/gen/Node.hpp.
In your case, if you have position(x, y) and know the node is a / is a child of Node2D then you could simply perform:
godot::Node2D* parent = get_parent();
godot::Vector2 pos = parent -> get_position();
godot::Godot::print(pos);