Just like Mike mentioned above it's not compatible. Specifically, the Galaxy S6, Galaxy S6 edge, Note 5, and Galaxy S6 edge+, Galaxy S7 edge & Galaxy S7 are the only phones that work with the new Gears VR.
For the V20 you're better off trying the LG 360 VR R100 or the cardboard. Daydream would be another great option, but the SDK is not exctacly tolerant to the LCDs at the moment. I would give it more time, besides OLED screens for phones are right around the corner. It's already been a success for apple watches, and with the 10th anniversary of the iphone coming up next year it would make sense for them to follow suit. I'm sure LG will have a similar phone to counter since they're the leading distributors of OLED at the moment (the displays for apple watches were actually supplied by LG Display).
Yea, at least it was free with my note 7 promotion. My fiance has an s7 so I guess I could use it with her phone, but she is not into tech.