I have been thru Gonzaga many times and there was no gas. This was true in October. If they are out try Rancho Grande Market across the street - great store BTW! They sell Guiness beer and a few other tasty choices! Or - drive to the houses on the beach - you can usually find a Gringo who sells gas out of a drum.
PMEX locations pretty close as follows
From RM148 at K60 to SF is 67KM = 42miles
From RM148 at K60 to Puertecitos PMEX is16KM = 10 miles
From RM148 at K60 to Gonzaga PMEX is about 90KM = 56 miles
Try Puertecitos and if closed go to SF where there is always gas.