If you are running a typical "consumer" router setup, such as 192.168.1.0, the "gateway" would be the router's address like 192.168.1.1 (typically). If you are using a different scheme, it would need to be adjusted accordingly. This depends on your router and how it is configured. Some use different addressing schemes. If you can access your router over your LAN with a browser, that would be the address.
The "netmask" for the "normal" type network would be 255.255.255.0, again unless you have multiple sub-nets. This masks out everything not on your LAN.
Although you didn't ask it, the "DNS" can be set to the Gateway's address (as above) or something like a well-known DNS server.
This is all done via the "Network" tab on the meteobridge. On your "System" tab, you can see what DHCP pulled. I would leave everything the same except for the IP address and copy them in one by one to the Network settings. If you like the IP address it obtained via DHCP, you could put that into the fixed IP address setting under "Network".
Greg H.