Mpg is worse than its inverse gpm, because it's not linear, and humans are really bad at comparisons of non-linear values.
Consider 3 vehicles. They get 10 mpg, 20 mpg, and 40 mpg. With these numbers, it's obvious which is the most efficient, but it's not easy to tell how efficient. For instance, will you get more fuel savings trading in your 10 mpg for the 20 mpg, or the 20 mpg for the 40 mpg vehicle?
Instead, if we look at those vehicles with their gpm ratings, we can quickly tell how much benefit we get from upgrading. Respectively, they are 10, 5, and 2.5 gallons per 100 miles, and it's much easier to see that going from 10 to 20 mpg is a much bigger improvement than going from 20 to 40 mpg.
Are you in the EU? My understanding is that those rewards are not available in the EU due to their much lower transaction fees. Admittedly, I'm not in the EU, so this is second hand knowledge.