We call it "Zulu" time because it's easier to identify a time on a briefing sheet using just 1 letter, "Z", rather than 3...ie "GMT" or "UTC". The navy picked the last letter of the alphabet to represent UTC time. The word used to represent "Z" in the phonetic alphabet is "ZULU". So on our briefings, we would just see the letter "Z" before a time. But when communicating on the radio, we used the term "ZULU". Hope that makes sense.