For me "400g of water a day" took me a while to interpret (I read 400 grams. That's less than half a liter, an amount a human cannot live on).

Because of that, I soon realized you must mean gallons, but I never saw that abbreviated to 'g' before. Is that common? Wikipedia and other internet sources only give me 'gal', 'gall', and 'gl'. On the other hand, 'mpg' for miles per gallon is perfectly normal.