Examples to convert from GDI to ImageSharp and back...

[equinox2k]

private SixLabors.ImageSharp.Image GdiBitmapToImageSharpImage(System.Drawing.Bitmap bitmap)
{
    var bitmapData = bitmap.LockBits(new Rectangle(0, 0, bitmap.Width, bitmap.Height), ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);
    var totalBytes = bitmap.Width * bitmap.Height * 4;
    var rgbaBytes = new byte[totalBytes];
    if (bitmapData.Stride == bitmap.Width * 4)
    {
        Marshal.Copy(bitmapData.Scan0, rgbaBytes, 0, totalBytes);
    }
    else
    {
        var sourceOffset = 0;
        var destOffset = 0;
        var byteWidth = bitmap.Width * 4; 
        for (var y = 0; y < bitmap.Height; y++)
        {
            Marshal.Copy(bitmapData.Scan0 + sourceOffset, rgbaBytes, destOffset, byteWidth);
            sourceOffset += bitmapData.Stride;
            destOffset += byteWidth;
        }
    }
    bitmap.UnlockBits(bitmapData);
    return SixLabors.ImageSharp.Image.LoadPixelData<Bgra32>(rgbaBytes, bitmap.Width, bitmap.Height);
}

private System.Drawing.Bitmap ImageSharpImageToGdiBitmap(SixLabors.ImageSharp.Image image)
{
    using (var tempImage = image.CloneAs<Bgra32>())
    {
        if (!tempImage.TryGetSinglePixelSpan(out var pixelSpan))
        {
            return null;
        }
        var bitmap = new Bitmap(image.Width, image.Height, PixelFormat.Format32bppArgb);
        byte[] rgbaBytes = MemoryMarshal.AsBytes(pixelSpan).ToArray();
        var bitmapData = bitmap.LockBits(new Rectangle(0, 0, bitmap.Width, bitmap.Height), ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);
        if (bitmapData.Stride == bitmap.Width * 4)
        {
            Marshal.Copy(rgbaBytes, 0, bitmapData.Scan0, rgbaBytes.Length);
        }
        else
        {
            var sourceOffset = 0;
            var destOffset = 0;
            var byteWidth = bitmap.Width * 4;
            for (var y = 0; y < bitmap.Height; y++)
            {
                Marshal.Copy(rgbaBytes, sourceOffset, bitmapData.Scan0 + destOffset, byteWidth);
                sourceOffset += byteWidth;
                destOffset += bitmapData.Stride;
            }
        }
        return bitmap;
    }
}