This is the case because GP4 is originally made for a 4:3 aspect ratio. If 16:9 is not to your liking then 16:10 (as the screen resolution 1680 x 1050 on a 1920x1080 HD-Monitor) may be a reasonable compromise .
Adding to the question. I have the game running in 16:9 but GPxPatch in game info is all squished to the left now. Mainly the time to car ahead and behind is what bother the most. Believe there is a way to adjust it for 16:9 but can't seems to find it.