I own .IO domain and I was able to set it as gccTLD in Google Webmasters Tool for at least last 6 months (but not when I registered it more than 2 years ago).
Not to mention my .io domain ranks quite well for my keywords across all national google front-ends for quite some time.
Not to mention my .io domain ranks quite well for my keywords across all national google front-ends for quite some time.