Google recently announced a beta test of a Google Voice Wi-Fi Calling feature: https://support.google.com/voice/forum/AAAAjq5-_rMaI7m1PrV21Y/?hl=en

The announcement notes that this feature disables compability with Obihai devices. Please note that it also disables compatibility with GVGW. It appears that enabling the Wi-Fi calling feature as described in this Google support article will permanently disable your ability to use that Google Voice account with GVGW. If this changes or any workaround becomes available, this post will be updated with new information.