According to the repair manual for my Ford Falcon AU Forte 1999

It says that the correct spark plug gaps should be:

4.0L 1.0 - 1.1 mm
5.0L 1.3 - 1.4 mm
4.0L Gas 0.9 mm

Two Questions:

1. Why do spark plugs electrode gaps have to be different?

2. What is the reason why the gap should be smaller for Gas as opposed to Petrol?

LPG is more dense that Fuel and requires a 'fatter stronger spark'

By closing the gap the spark is thicker and travels less.

Also as LPG burns dry, the combustion chamber gets hotter and it is best to run a longer electrode ie: colder plug.

Google sparkplugs or something Im sure there's a few hundred pages describing why there's different gaps.