Form.Opacity doesn't work for you?
That's just how the GroupBox control works. The Background property of that control includes the area that your screenshot points to. If you wanted to do a workaround for it, you should set the GroupBox background to be transparent as well, and draw a white box behind it, encompassing only the area you want to be white.
What is the GroupBox's parent container? Is it a Form or a Panel? Is that element also set to transparent? As a test I made a GroupBox and placed it on top of (inside) a Panel. I changed the background color of the Panel to red and the background color of the GroupBox to transparent and those ares of the GroupBox are properly transparent. My suggestion is to look at the parent container.
Also the GroupBox label may become hard to read in some cases, once the top strip is transparent.