Yes, the Simonics GVGW supports TLS encryption, but I doubt it does what you want it to do. TLS encrypts the SIP channel, which registers the device and sets up the call. However, the actual voice traffic (Media) is handled by the RTP protocol using a different port. So, with TLS, your conversations can still be easily tapped, although it would be quite difficult to decode the phone number you placed the call to.
As far as I know, the GVGW (along with the vast majority of VOIP providers) do not offer secure RTP, which is probably what you want. And this surprises me, as I think it would be a popular option if someone did offer it.
(I've been meaning to test whether
voip.ms IAX2 offering supports encryption. Does anyone know?)