Gun graduated with an upper second-class degree, then took a job as an assistant English teacher with the JET program in Hiroshima, Japan. She left teaching in 1999, and after some temporary jobs, finding it difficult to find work as a linguist, Gun applied to GCHQ in 2001, after reading a newspaper advertisement for the organisation. Gun had previously been unaware of GCHQ, later saying that "I didn't have much idea about what they did...I was going into it pretty much blind. Most people do."