Have you installed the utilities for Unity? [According to the docs][1]According to the docs, 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]here.
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
