No man… 
You presumably have an ethernet switch/hub/router somewhere? Your GX already connects to it? All you need is for the Fronius to be connected to this same switch/router, either via a cable, or via Wi-Fi. This doesn’t require running another cable to the GX device (but it may require running another cable to whatever networking infrastructure you have in the house.
Because it keeps the grid meter at zero… it works even with unsupported PV-inverters. If there is a PV-inverter there (say a Solis that is not supported), the GX device is still aware that something is making extra power. It completely screws up the calculations as to how much loads you have on the input side (because it cannot subtract that part, it doesn’t know about it), but it works properly in all other respects, using that power to charge batteries etc.
Aaah ok I get you. Fronius and Venus does not have to connect to each other directly then. The Venus connects via wifi to a router and then so must the Fronius.
Nothing else. Just the bare basics. No fancy home automation nothing.
But if this is a Fronius (that is supported from my understanding) then the GX will no exactly what is going on?
Yes. Anything that supports sunspec, the GX will know there is a PV inverter and can account for the energy. But if you want to limit grid feed-in, then it must be a Fronius or an ABB/Fimer PV-inverter, those are the only ones supported.