It was a random mutation 1st hatched out by Ron Tremper.
Click the following link to Ron Tremper's site...
http://www.leopardgecko.com/giant-genetics
...It is his Giant genetics page, and gives a brief history on it... It will answer any of your Giant questions you may have.
...and then check this one out... It's his "Giant FAQ" page... Any other questions you may have will prolly be answered there... If you have one not listed there, then just ask anyone on here, and i'm sure we'll be able to help ya! If someone doesn't have the answer, someone else surely will!!
http://www.leopardgecko.com/giant-faq
It was likely the super extended captive breeding seasons that caused him to stress out, but honestly 6 years is pretty awesome for a lizard.
More than likely, the answer to why Moose died is against TOS...