Asides

It seems to be a need to know the used ports by the Remote Desktop RD Gateway. Find a short overview bellow:

Internet –> Gateway WAN NIC:

TCP: 443
UDP: 3391 (You have to enable UDP on the RD Gateway)

Gateway LAN NIC –> Session Host Servers:

TCP / UDP: 3389

Gateway LAN NIC –> Connection Broker Servers:

TCP / UDP: 3389
TCP: 5504
TCP: 5985

Gateway LAN NIC –> Domain Controllers:

TCP / UDP: 88
TCP: 135
UDP: 123
UDP 137
TCP: 139
TCP / UDP: 389
TCP: 3268
TCP / UDP: 53
TCP / UDP: 445
TCP: 5985
TCP Dynamic Ports (NTDS RPC service )

Connection Broker Servers –> Gateway LAN NIC

TCP 5985 (WS-Management and PowerShell Remoting)
TCP: 3389 (Remote Desktop)

Let me know if this helps.
Cheers,

Al