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

Today,a doughboy or GI may be called a grunt.

VOA: special.2010.03.07

Soldiers often say, We GId the place. And when an area looks good, soldiers may say the area is GI. Strangely,though, GI can also mean poor work, a job badly done.

VOA: special.2010.03.07