GP4 does not lists the actual resolutions your computer can do, it has only a set of predefined values, so to use a 16/9 aspect ratio you need to edit the f1graphics.cfg file, in the install folder.
Edit the horizontal and vertical resolution values that are near the top of the file (with [RELEASE] prefix).
Note that once you set a value that is not in the predefined list, the game will display 800x600 in the option menu. That won't affect the resolution in the game, and the value you have written won't be overriden unless you change the value in the menu.