As I understand it, in aviation and some armed forces, the term Zulu time (denoted by the letter Z) is used to refer to GMT. What is the origin of this? Why the use of "Z"/"Zulu"?
It's all explained in this Wikipedia article. Basically, nothing to do with Zulus in Africa - it's just that in the NATO phonetic alphabet (and amateur radio) contexts, the word for Z is "Zulu".
In the context of Coordinated Universal Time (UTC), Z stands for "zero hours", meaning the offset from Greenwich Mean Time (GMT) is zero. Effectively, Zulu Time is GMT.