I was annoyed when fiddling around with Gamepad Support for this game on a mac, but I finally found a solution:

All I had to do to have the gamepad recognised as the player 1-gamepad was to go to System Settings -> Game Controllers, manually add Grid Autosport.app and enable the Xbox Controller Compatibility-Setting.

Without that setting, it would always turn turquoise (as "gamepad #2") and wouldn't show up unless I started Splitscreen-Mode.

Hope this helps :) (Might also be important for Wheel-Support on the Mac. Haven't tried it yet.