Gasoline quality tends to vary by region, depending on the refinery/pipeline situation and whether it is a conventional or reformulated grade of gasoline.
While someone in Georgia may report excellent mpg in a 94 Camry, running Shell 87 octane conventional gasoline, this same car may post a lesser mpg in the Chicago/Milwaukee market where the Shell branded gas is supplied thru a ConocPhillips refinery and is reformulated with ethanol as the oxygenate.
Since Gulf only exists nowdays as a marketing brand, the stations are most likely supplied with whatever ExxonMobil, Marathon, Citgo, BP, etc. gasoline that the tanker trucks are daily pulling from the bulk tanks at the distribution terminal.
The gas that your friend at work claims to give this "extra mpg" could be the same that you are running in your daily drive, it's difficult to pin down.