Not sure this is a bug, and not sure if this X10II only.
I just spend an evening trying to install and get Gboard running. I could install and enable it, but it didn’t show up when I clicked e.g. the text field in WhatsApp or URL bar in Firefox. When I switched to Jolla keyboard (uh, aliendalvik keyboard or something, can’t recall) it works as expected. Then I tries SwiftKey, but it behaved exactly the same way.
I then tried messing around in Android Settings app, and stumbled upon “show virtual keyboard when physical keyboard is connected” and when I enabled that option, Gboard started to work, even though there is no physical or Bluetooth keyboard attached.