My mate has one and in 18months it grew from 4" to 20"! When I get round to donwloading a vid from my phone to youtube, I'll post it here for you to see it. He calls it Mr G!
He sold his tank a while back as he was going to emigrate. That didn't happen so he bought another tank and rang the new owner. He didn't really want to sell but my friend offered a lot of money for it, so he sold it back!
My LFS had a 17yr old GG in a display tank, that was about 24". Sadly died early this year. He had that from a baby as well.