I just bought a gear fit 2, and installed Gear app on my note 3, I have the very latest version.
The auto unlock option is there and it worked yesterday.
Today, it was grayed out and like others said, I would go through the motions of turning it on but it would still be disabled.
An earlier post in this thread mentioned the fix, I guess some of you overlooked it.
If you use any apps that install credentials, then auto-unlock will be disabled. You can't even change your standard screen unlock to swipe.
My banking apps, my VPN, every time I use them, it creates a credential and does this.
Go into security, "clear credentials" and it solves the problem. Auto unlock with the gear fit 2 works again. Don't worry, whatever app placed the credential will simply create a new one as needed.
I have been clearing credentials so I could have 'swipe to unlock' for about a year now, so I can promise you, it doesn't hurt anything. I just didn't realize it would disable it on the gear fit 2 also.