Many people seem to answer this question, including the ones who've read The Hitch Hikers Guide to the Galaxy
Well, Gagravarr (spelt Gargravarr in some versions) is one of those lesser known HHGTTG characters. One of those who you remember when someone reminds you the sections involving them, but otherwise not.
Gagravarr is Custodian of the Total Perspective Vortex on Frogstar World B. He guides Zaphod into the Total Perspective Vortex (hence the image on the front page), after the unfortunate incident involving the government and removing the Guild offices. He is a disembodied voice, as his body is of partying somewhere.
At the time that I was trying to pick an online persona, I happened to be listening to HHGTTG. Since many of the more obvious names seemed to be taken, I grabbed the CDs, and started looking down the cast list. Gagravarr was the first not obviously taken one I came across, so I picked it. Simple as that.
For those of you who haven't heard the radio version of the Hitch Hikers Guide to the galaxy (shame on you!), or those who haven't heard it in a while, there's the following sound clip:
The clip is from Fit the Eighth (2nd episode of the Secondary Phase), and is set as Zaphod arrives on the Frogstar to face the Total Perspective Vortex.
If you want to listen to more like that, you'll have to track down a copy of the radio version. The BBC are currently selling all the episodes of the radio version on an mp3 cd - see here. The other option is to get the audio CDs of it. There used to be two box sets, one for each series, but they're currently hard to come by. Check ebay / your favourite second hand music shop. They were published by BBC Worldwide Ltd, and have ISBNs of 0-563-38974-5 and 0-563-38979-6, if that's of help.
(Gagravarr doesn't occur in the Tertiary or Quandary phases of the radio series. That said, they're both really good, so go listen to them! They're also much easier to get hold of on CD than the first two, so you have no excuses!)