Whom did Gatsby fight for in the war?It was in chapter 4, 5, 6 or 7. But I can't remember. :( Please help

In Chapter 3 there is a conversation between Jordan Baker and her friend Lucille about which side Gatsby actually fought on in World War I. There is a mystery surrounding whether he was an American soldier or a German spy.

"I don’t think it’s so much THAT,” argued Lucille sceptically; “it’s more that he was a German spy during the war.” One of the men nodded in confirmation.  “I heard that from a man who knew all about him, grew up with him in Germany,” he assured us positively.  “Oh, no,” said the first girl, “it couldn’t be that, because he was in the American army during the war.” As our credulity switched back to her she leaned forward with enthusiasm. “You look at him sometimes when he thinks nobody’s looking at him. I’ll bet he killed a man.” (Fitzgerald)

Gatsby is a difficult man to actually know, part of the reason for this is because he recreated himself, he became Jay Gatsby.  He was born James Gatz from North Dakota who endured a poor upbringing by dreaming to be rich. 

Another clue to Gatsby military service is the fact that he was stationed in Louisville before being shipped overseas, that is when he met Daisy Buchanan the woman who inspired him to seek and acquire wealth.