And in many places M stands for thousand, from Latin "mille" (as well as the Roman numeral). That doesn't keep people from counting in kilo-, mega- and gigadollars. Still less confusing than dealing with short vs long scale numbers in translation.
I wouldn't immediately parse that abbreviation in the US--but if I did I'd parse it as G being shorthand for grand. More commonly 2Gs would be used for 2x the normal force of gravity--i.e. you weigh twice as much.
As with many things I never expect others to be respecting arbitrary style guides when they write something. (Same reason I wouldn't use semi-weekly or biweekly.)
Also G (uppercase) doesn't have units that result in G$ being a plausible quantity even if you could combine number-unit quantities and use like a unit like that (which you can't for this exact reason): dollar-cubic metres per kilo per second per second.