bunch of folks - a certain number of peoplebunch of folks who were changing the petrol pumps in some way. Maybe they were changing the gauges so that they said they pumped out more petrol than they really were, and the bad guys pocketed the difference in money.
gauges - devices that show how much [petrol, in this case] has been dispensed/used/served
pocketed the difference - took the difference [between the amount dispensed and the amount shown on the gauge] and added it to their own possessions ('pocketed' it).
So: A number of people were changing the petrol pumps in some way [maybe making the gauges show 10 litres when they had really served only 9 litres], charging the customer for 10 litres, putting payment for 9 litres in the till, and putting the value of 1 litre in their own pocket.