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

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