GIJoel, like most things it can be done, but involves some stuffing around. On the factory VT/VX setups, the LPG light uses pin 20 on the dash cluster's wiring plug. On non-LPG models this pin 20 is not used/vacant. BUT it's not as simple as wiring in pin 20 alone - the light itself is activated thru Tech-2 and will only work on the factory LPG setups AFAIK.
I had a SpintGas mixer system fitted onto my VT in '07 (upgraded to vapour injection in '10). While i had the old system, i fitted a factory momentary LPG switch in the dash, connected my LPG tank to the dash cluster's fuel gauge using a special gas tank sender (so both fuels used the one gauge etc).
I also modded the back of my dash cluster to get the LPG light to work. To do this, i had to carefully cut the tracks in the circuit board around the LPG lamp, then wired in two wires. One was ground, the other was +12V from the LPG solenoid (i.e. the +12V is only present when car is running on gas).
It worked well, but as i said it's a lot of stuffing around - depends how much time, patience, determination etc you have.
But once i had the vapour injection fitted, i decided to stop using the LPG light and combined fuel gauge. I've found LED gauges are better for gas (they seem to show a more "accurate" reading than a factory petrol gauge), and in the end i just decided having the LPG lamp was nice but unecessary. Plus i made a mess of the back of my original dash cluster modifying it (my soldering/electrical skills are basic) so even though it worked how i wanted it to, i ended up replacing the dash cluster with another that had the same km's and un-modified.
There's nothing wrong with wanting the lamp to work though so if you can be bothered, go for it.
Btw an Aussie company called Apexus used to fit their own kit to VT/VX which also turned on the LPG light, not sure if this kit is still available though. Checking out this link may be of interest:
www.apexus.com.au/document/kits/qk103.pdf)
Here are some old LPG threads i posted in on this forum that also might be of use:
http://forums.justcommodores.com.au/vt-vx-holden-commodore-1997-2002/157350-lpg-conversion.html
http://forums.justcommodores.com.au...997-2002/155629-cluster-needle-alignment.html
http://forums.justcommodores.com.au...45641-factory-fitted-lpg-switch-location.html
http://forums.justcommodores.com.au...97-2002/107382-converting-gas-lpg-switch.html
http://forums.justcommodores.com.au...7-2002/89187-factory-lpg-button-location.html