"F1 cars have a reverse gear, but these are designed to satisfy the regulations rather than being of much practical use. Most teams build avery small and flimsy reverse gear on the outside of the gearbox to help keep the weight of the gearbox down, as reverse gear is seldom used (apart from the odd trip down an escape road at Monaco...)"
Some race cars don't have a reverse gear in the gearbox, but use a small electric motor for reverse duties which is operated via a button. This is a common solution in BEC's (bike engined cars).