On my 09 I have it set at 10 for one rider (215 lbs) and 20 for 2 riders. With 2 up and luggage I set to the max.. Settings go to 25 I believe on the GL1800. Don't remember how high the settings go on the GL1500.
I will assume you are asking about your 1500, correct?
If so, pump the rears up to about max. Sit on the bike. Have someone watch the rear fender tip, even use a ruler to the floor. Start letting air out slowly, a little at a time.
When the fender starts to drop, stop! Read the gage-that's your setting.