I did not use gear display but think you will just need to input tyre size as it works by wheel speed again RPM i believe.

If running on the T6 it is very simple to wire up as will be the same as mine on T2.

Think it was:

Ignition live
Earth
ECU Tx to the AIM Rx and thats it.

You will need to alter a setting in the ECU as well but that is easily done