You have to convert the gdi+ image to a gdi+ bitmap and then to an OpenCV matrix. There is no easier way to do the first conversion than creating a bitmap and painting the image into it, as far as I know.

To perform the second conversion (Gdiplus::Bitmap -> cv::Mat), note that Mat constructors order their parameters rows then columns so that is Bitmap height then width. The actual memory layout is row major, however, so we can just copy the data, but there is no need to do the actual copy yourself. You can use one of the Mat constructors that will wrap existing data without copying and then force it to copy by calling the clone member function.

The other trouble here however is that gdi+ supports, in theory at least, loads of exotic pixel formats so to handle them all would be a chore. The following handles the basic case:

Gdiplus::Bitmap* GdiplusImageToBitmap(Gdiplus::Image* img, Gdiplus::Color bkgd = Gdiplus::Color::Transparent)
{
	int wd = img->GetWidth();
	int hgt = img->GetHeight();
	auto format = img->GetPixelFormat();
	Gdiplus::Bitmap* bmp = new Gdiplus::Bitmap(wd, hgt, format);

	if (bmp == nullptr)
		return nullptr; // this might happen if format is something exotic, not sure.

	auto g = std::unique_ptr<Gdiplus::Graphics>(Gdiplus::Graphics::FromImage(bmp));
	g->Clear(bkgd);
	g->DrawImage(img, 0, 0, wd, hgt);

	return bmp;
}

cv::Mat GdiPlusBitmapToOpenCvMat(Gdiplus::Bitmap* bmp)
{
	auto format = bmp->GetPixelFormat();
	if (format != PixelFormat24bppRGB)
		return cv::Mat();

	int wd = bmp->GetWidth();
	int hgt = bmp->GetHeight();
	Gdiplus::Rect rcLock(0, 0, wd, hgt);
	Gdiplus::BitmapData bmpData;

	if (!bmp->LockBits(&rcLock, Gdiplus::ImageLockModeRead, format, &bmpData) == Gdiplus::Ok)
		return cv::Mat();

	cv::Mat mat = cv::Mat(hgt, wd, CV_8UC3, static_cast<unsigned char*>(bmpData.Scan0), bmpData.Stride).clone();

	bmp->UnlockBits(&bmpData);
	return mat;
}

cv::Mat GdiplusImageToOpenCvMat(Gdiplus::Image* img)
{
	auto bmp = std::unique_ptr<Gdiplus::Bitmap>(GdiplusImageToBitmap(img));
	return (bmp != nullptr) ? GdiPlusBitmapToOpenCvMat(bmp.get()) : cv::Mat();
}