The Dictionary of Soldier Talk says GI was used for the words galvanized iron in a publication about the vehicles of the early twentieth century.

VOA: special.2010.03.07

you do certain amount of years in the military, and then you get what's called a GI bill,

你先在军队服役几年，当你退役的时候，你就能获得美国军队的补贴,

拿全额奖学金 - SpeakingMax英语口语达人

Over the years Congress has passed legislation like the "GI Bill of Rights."

VOA: special.2010.11.08

Some students of military words have another explanation of GI.

VOA: special.2010.03.07

But growers are hopeful that if Kampot pepper can earn GI protection next year, the province can regain its status as one of the world's premier pepper growing regions.

VOA: standard.2009.04.16