It's B3ta's instant message system
If memory serves, Gaz came from someone posting 'Gaz IM me' and then vanishing, never to be seen again.
It became a bit of a text meme and when the IM was set up, everyone just automatically called it 'Gaz'.
(
Flowerpot No longer has the vapours thanks to DTH,
Sun 23 Oct 2011, 0:10,
archived)