G.I. is a word that is used to describe something or someone who is part of the United States Army. The term originally stood for galvanized iron, but is commonly used to stand for government issue. Another use of the word, which is incorrect, is General Infantry and this meaning of the word G.I. is incorrect because it has never been used officially by the United States Army.
The word G.I. originates from shortly before WWI and has developed multiple meanings besides galvanized iron and government issue. One of these other uses of the word was G.I. Can which was meant to describe WWI German artillery shells. The word G.I. that meant government issue was often popularized by newspapers and comics.