You will probably find that many people who work for GCHQ have no idea what OTHER parts of GCHQ do.
When we were code breaking at Bletchly Park during the war many temporary huts were built. These were just given numbers, hut 1, hut 2 etc.
People were not allowed to dicsuss what went on in other huts and often people had no idea what was going on in the hut next to them.
The same applies to GCHQ (except we are talking offices rather than huts)
There is some jobs going, why not try to get one and learn what they do from the inside. :-)
http://www.gchq.gov.uk/recruitment/intell.html