When using GraphicsUnit enumerator value on native C++ , we should use Unit enumerator.
It's declared at GdiplusEnums.h as follows.

enum Unit
{
UnitWorld, // 0 -- World coordinate (non-physical unit)
UnitDisplay, // 1 -- Variable -- for PageTransform only
UnitPixel, // 2 -- Each unit is one device pixel.
UnitPoint, // 3 -- Each unit is a printer's point, or 1/72 inch.
UnitInch, // 4 -- Each unit is 1 inch.
UnitDocument, // 5 -- Each unit is 1/300 inch.
UnitMillimeter // 6 -- Each unit is 1 millimeter.
};