Here's the historic origin of the term "GI":
It all started with 18th century physiologist and physicist Luigi Galvani who studied impact of electrical forces on muscular movements. People began to use the term "galvanize" to refer to stimulating muscles using electricity (as well as metaphorically stimulating mind), and also to coating iron articles with zinc using electricity to prevent rusting.
GI was originally an abbreviation for Galvanized Iron, a US army clerks' term for items such as trash cans (which are galvanized), but later the abbreviation transformed to stand for "Government Issue"--all articles issued in conformity with US military regulations or procedures. Still later the abbreviation transformed to refer to US soldiers themselves.... and GI Joe was born!!