I'v installed first beta of iOS 10 today. GCA freezes on main screen, however, if you tap any other tab as soon as App starts, you will be abble to sync your watch and use all other features of GCA. Only the main screen doesn't work and hangs the App.
So, if you really like the main screen of GCA, you propably shouldn't test Apple'a latest iOS beta and wait for GCA update.
Tested on iPhone 6 and Fenix 3.