Is there a way to expose C++ enums into gdscript? Or more precisely how is it done in the engine?

GDScript has enums. What is it you are trying to accomplish?

https://docs.godotengine.org/en/stable/getting_started/scripting/gdscript/gdscript_basics.html#enums

Enums don't exist post-compile, they just get converted to numbers. They are effectively just a convenience for the programmer and are an abstract way of representing integers.

When you make a DLL/SO/DYLIB you are compiling your C++ code down into the dynamic library that you then connect to Godot. Your enums no longer exist at that point. You can make your own enums on Godot's side that mirror your enum values in C++, I suppose.

In the engine code, for example the Button node, you have this in the header file:

class Button : public BaseButton {
	GDCLASS(Button, BaseButton);
public:
	enum TextAlign {
		ALIGN_LEFT,
		ALIGN_CENTER,
		ALIGN_RIGHT
	};
	//...
	static void _bind_methods();
	//...
};

// This is needed for some C++ casting stuff
VARIANT_ENUM_CAST(Button::TextAlign);

And then in the cpp file, TextAlign is bound using the following macros, inside the _bind_methods function:

	BIND_ENUM_CONSTANT(ALIGN_LEFT);
	BIND_ENUM_CONSTANT(ALIGN_CENTER);
	BIND_ENUM_CONSTANT(ALIGN_RIGHT);

You can have a look in these files: https://github.com/godotengine/godot/blob/master/scene/gui/button.h https://github.com/godotengine/godot/blob/master/scene/gui/button.cpp