It's not really any different to running on petrol. You may experience a slight drop in power, but on the two Range Rovers I have on LPG the difference is so small as to be insignificant. Most LPG systems are designed to start on petrol then switch to LPG once the engine is running. On a lot of them this is done automatically once an pre set engine speed is reached. It's best for this to happen when the engine is not under load so the procedure is: Start engine on petrol, blip throttle to the point where LPG sweitches on, drive off.
If the system is manually switched (ie by a switch on the dash board) the same advice applies. Don't switch to LPG when under load - it may cause a backfire. This is due to the mixture leaning out in the time delay between switching petrol off and the gas getting to the combustion chambers. Rev the engine and as the revs drop switch to LPG.
TRH