I've had a few people ask me about it, so I'll put the info here so everyone can see it in one place.
I do not know what happened to GreyFireFox or why he shut down his DeviantArt gallery. The last I heard from him was on February 23rd, 2013. He emailed to let me know that progress on his Krystal rig version 1.5 was proceeding, though slowly.
I've tried contacting him via email and FurAffinity PMs with no response. That was several months ago.
And while I'm at it, there are at least 7 renders of his that I don't have. If you have one of the images below in full resolution, please email me so I can post them on his page (don't worry, he gave me permission a long time ago).
Update: Thanks to Alexander and Vukasin, who both sent me the missing renders. I have added them to GreyFireFox's page.