I am able to view the gallery using the provided registry keys in VS 2013. Manually adding the gallery through the "Change your Extensions and Updates settings" link in the VS 2015 Extension manager results in the following message when attempting
to see available online updates:
"Cannot check for updates because the following error(s) occurred while trying to contact the server: Could not determine the protocol used for this gallery. The URL must point to an Atom Feed or Sharepoint List configured for hosting Visual Studio Extensions."
I'm testing on my local machine with the following registry key file contents:
@="http://localhost/InmetaGallery/GalleryService.svc" "Priority"=dword:00000064 "Protocol"="VSGallery"
Visual Studio Gallery"