I found this information in FAQ GSC2013
Unfortunately, this is not working for me in GSC2013. Someone knows the correct ports ?
Which ports do I need to open on a dedicated server in order to host?
TCP port 34747 must be opened. In addition to this, the UDP port numbers are listed in the multiplayer.ini as 34597 and 34697 by default need to be opened. You need to open from these two UDP numbers upwards for each server you have open.
Unfortunately, this is not working for me in GSC2013. Someone knows the correct ports ?