Welcome to the APEXUS Pty. Ltd. World Wide Web Site (under 'Gauges')
The GA100 is what I have. I bought it from a wrecker but it wasn't easy to find one. I didn't try to buy it new. There are restrictions on buying LPG components without an installer's licence but that may not extend to the gauges. It may therefore be worth ringing any of the suppliers of LPG components.
Wiring this one up was easy: one power wire, one earth wire and the third wire is earthed through the tank gauge (which incorporates a gauge sender) = wire from LPG dash gauge to one terminal on the tank gauge and the other terminal on the tank gauge goes to earth.