How Many MM Should The Spark Plug Nail Gap Be?

I use LPG in my vehicle, while installing the master spark plug nail range 6.5, the vehicle started to have misfire and fuel problems. Later when I took it to another master, he said that such things should not be done in the peugeot group, especially in the French cars, and found the original values from the chassis number and made the spark plug nail intervals of 8. The lower revs of the vehicle strengthened.