What does "gearbox failure" mean?

Gearbox failure message on instrument cluster means that your transmission is in "Limp" mode and will have only second and reverse gear. You have to bring your Jaguar to specialty repair shop that will be able to scan transmission control module and retrieve error codes. There is many possibilities, from simple electrical problem like defective solenoid, sensor all the way up to worn / defective clutches causing incorrect gear ratio - slipping. In any case having warning light on dashboard, proper diagnostics is a key to successful and accurate repair.

It really depends on what code is setting the transmission into limp mode to know