You should use a ballon, means a little passive devoce that have a RJ45 conector on one end and a coaxial BNC conetcot on the other.
They should not be too expensive, and inside the device there is a impedance matching component that will do the conversion from 120 to 75 ohms.
This device looks just like a oversized coaxial conector with a RJ45 conector on the other end.
There is no much loss if you somehow managed to connect the coaxial to the gateway, but there is another issue. I think that the gateway is a balanced input anf the provider is unbalanced (one hot wire in reference to ground)
It may work, but you will be a good antenna for any discharge coming trough the phone lines.