Unfortunately Vixen no longer supplies a GOTO upgrade for manually controlled mounts such as the GP2 and GPD2.
The Star Book S controller set was discontinued in 2012.
Some third party solutions such as the Skywatcher EQ3 adn EQ5 GOTO upgrades can be fitted to some Vixen mounts. However, these motor sets are not supplied or supported by Opticron. Instead you should contact your local astronomy dealer for information and pricing.