When I play GP4 with my wheel, the space bar pauses.
Accelerator becomes A, and scroll up in the menu
Brake is z, and scroll down in the menu
Fire (or gear up) becomes the space bar.
So to leave the pits i have to press my brake and gas pedals to highlight 'leave pits' then hit gear up on my wheel to select.
If you are not using keyboard then try these out.
If you are, then i don't know because if i go to controls and keyboard it reverts to default without me having to do anything.
As said, just check your keyboard mappings.