Tim is probably correct, but there is a slim chance he is overstating the complexity.
What is for sure is that you need to purchase more Racepak hardware. The usual advice would be a Universal Sensor Module. This will expand your data capacity by adding four possible additional external channels.
Also for sure is the need for programming - it will not be a simple "plug and play" style procedure.
However, it is not necessarily complicated.
The complexity will depend on the signal that you can extract from the GSX hardware.
I've successfully achieved the result you might desire, which given the nature of the bike style sequential gear shift is certainly desirable, being a display of what gear is selected when the vehicle is stationary. However, I was working with bespoke race hardware, rather than mass produced.
The best outcome that you might hope for, assuming the given of 1st, 2nd etc displaying 1, 2... is that neutral displays 0. However, this will not necessarily be possible without custom electronics. Most production gear indicator displays use a microprocessor and digital logic. Thus, the output you can pick off may not be nicely analogue and suitable for direct input to the Racepak module.
The first step is to find and measure the output...