I only use GV for outbound failover and do not need an additional gateway for this.
Other users seem to have no significant complaints with the GV gateways. However, they are an additional point of potential failure. POTS was never as fragmented as are so many DIY VoIP implementations.
As for faxing through a GV gateway, it should be no better than faxing through GV, which as you noted in your original post is not perfect. I believe GV is for voice calls only, so I would not rely on GV for fax/data comms, gateway or no gateway.
OE