I always wondered where the phrase came from. I couldn't figure out two words with G in them that worked and I couldn't find anything on the internet. I figured it came out of some marketing book or consulting company and the phrase just got passed around.
It was in a WWII book I am reading that I found the answer:
G-1 is a U.S. Army staff designation. It signifies the staff officer responsible for operations at a divisional or corps headquarters. Likewise a G-2 was responsible for intelligence, a G-3 for supply.
So like ASAP and snafu, it's an army expression, in this case just shorthand for "intelligence".