How do you fix a 1998 jaguar xk8 gear box fault?

If it's just the light that comes on, a Jaguar dealership should be able to reset the onboard computer and remove the error state which triggered the light.

A gear box fault can sometime occur due to a glitch in the computer software for the gearbox. If so, resetting as outlined above should sort you out.

If on the otherhand there is a 'physical' problem with the gearbox the error state may return. Your best bet is to have the box looked at.

It's well known that catastrophic gearbox failure occurs around 96k miles requiring a new (or reconditioned) box.

Hope this helps, and good luck