You forget, due to us thinking base 10, dividing by 100 is not computationally intensive, just mentally shave off the last two digits. It's an operation that doesn't take any time. It's like in a computer where a divide is normally pretty computationally intensive, requiring several CPU cycles. But dividing by two is a special case since computers think binary, so it requires only a right shift of the bits (if you don't care about remainder).
Say I drive 15,000 miles per year, car gets 4 g/hm, that's 4*150=600 gallons per year. Say I drive 15,000 miles per year, car gets 26 mpg, that's (opens the calculator to do division) 576 gallons per year. With MPG you have to divide by some number, usually one you can't quickly do in your head. Gallons/100miles has already done that division for you, leaving only multiplication.
But this is all about the number on the dealer sticker being more meaningful, and gphm definitely is for that purpose.
Dividing total annual miles driven by MPG is one easy calculation. When I track my mileage, MPG is also very easy to calculate since I just divide my miles driven since the last fill-up by the number of gallons required to fill the tank to get MPG without then needing to convert that number to a per 100 miles base.
The bottom line is that the two numbers are just different ways of expressing the same mathematical data, so to say that one is "stupid" and the other is "more meaningful" is actually a pretty stupid statement in itself (this is directed toward the idiot cited in the article and not to any commenter posting on this thread). People who are not mathematically smart enough to figure out that an increase from 10 mpg to 20 mpg will save them more gas and more money than an increase from 35 mpg to 50 mpg would will be just as confused and inept at figuring out whether an improvement from 10 to 5 gphm is better or worse than an improvement from 2.857 to 2.000 gphm. (It really doesn't matter how easy it is to calculate when you are a mathematical idiot to begin with.)
Either number can be meaningful and helpful for consumer information and for performance metrics (which should be entirely left to the marketplace rather than dictated by government). Since a standard has already been established in the marketplace of measuring the car's fuel efficiency in terms of miles per gallon, changing this measure would be counterproductive (and costly) in the short term and would probably have negligible -- if any -- benefit in the long term. If the marketplace decides -- response to demand in the marketplace -- to change the way fuel efficiency is expressed, then that is fine. But if some bed-wetting anti-consumerism bureaucrat decides that the way we express fuel efficiency here is "stupid" and it would be better if we were more like the Europeans, and as a result regulations are imposed requiring that auto manufacturers and dealers have to revise the way the numbers are calculated and displayed on the stickers (and I guess that means all those on-board computers will need to be reprogrammed so they comply with the new government edict) all that is accomplished is that consumers are given a DIFFERNT piece of inaccurate information that most of them won't know what to do with in the first place, but we will have successfully increased costs for everybody. After all, if we can spread more costs around for everybody then that's better, isn't it.