BSFC for an outboard is about 0.5 # of fuel per HP per hour for steady state loading (your motor may be different) 2-strokes are closer to 0.55-0.8 depending on design lets pic 0.6 for your 2-stroke
if you run for 1 hour, 60 x .6 = 36 # of fuel, or at 6.82# per gallon, you would have burned...