I don't know much about the RB25DET but with some engines LPG causes cylinder head wear and valve seat issues. Petrol lubricates the combustion chamber, which is something that LPG does not do, so if your running straight gas then you will need to do more maintenence, more often. Plus the fact that is has about 22 mj/litre compared to around 33 mj/litre for petrol means that you need to burn more for the same power.
My mothers Falcon had LPG on it, it was gutless on LPG, I don't care what any body says if you run duel fuel you will have less power on LPG.
For a car such a Skyline, which is not the thirstiest vehicle around anyway, I wouldn't bother with it, you might save a bit of money, but if money was the primary factor you would be driving a Corolla.
/end rant