Have you installed the utilities for Unity? [According to the docs][1], the long click on the back button should **always** show the GearVr Menu. But, if it is not working, there is also a script (OVRPlatformMenu.cs) in the utilities that does that. You can find more info about the script [here][2].

Good luck

 [1]: https://developer.oculus.com/documentation/unity/latest/concepts/unity-ovrinput/#unity-ovrinput-gear-vr
 [2]: https://developer.oculus.com/documentation/unity/latest/concepts/unity-utilities-overview/#unity-scripts-scenes