As far as I was aware, GQ's only came out with the one tank from factory, around 85L (at the rear). This could be different for diese/ute variants. Certainly my GQ only had the one tank until I installed the aux tank.
Yeah, my Aux tank is an LRA, so 68L is probably right, add in fuel in the line etc, and it probably feels like a bit more.
For Gas, normally the rear tank is removed, and then they add an aux tank in the centre - depending on the gas mob these can be 30/40/50/70L units from what I've seen.
As for converting to LPG, I do too many kms offroad in the high country, often switching between the tanks well before I get to a servo, so while it would save me money, I'd be losing a lot of my off road 'range'. Aside from which, too many servo's still don't have LPG. Would be nice for the local trips though.