Not available yet; and probably never will be. All the effort is going into Win 10. Canada's 635's are listed as having Denim available. But not the AT&T 635 in the USA. In fact the very few phones on US carriers have Denim available. All of a sudden AT&T has forgotten about Windows phones, where as they used to be the main supporter. Really feel bad that Verizon has passed AT&T as far as support for Denim. How bad can things get. Don't feel too bad though. I have been waiting for the upgrade for my Moto G lte for quite awhile also. Google is no better.
What is the version number you have now? Did you do "slow track" or "fast track" download?
I did it for my 830 and 635 on "slow track" in the beginning of May, and got version 8.10.14219.341, but this is not GDR2... it was Windows 10 after that, no GDR2, I wonder if they have GDR2 now (version 8.10.15116.125)?