The ports SRCDS officially requires are:

27015 TCP/UDP (game transmission, pings and RCON)  Can be changed using -port on startup
27020 UDP     (SourceTV transmission)              Can be changed using +tv_port on startup
27005 UDP     (Client Port)                        Can be changed using -clientport on startup
26900 UDP     (Steam Port, outgoing)               Can be changed using -sport on startup

SRCDS has also been spotted opening connections on 27005 and 51840 UDP, but these may be outbound only.

https://developer.valvesoftware.com/wiki/Source_Dedicated_Server

sudo ufw allow 27015/tcp
sudo ufw allow 27015/udp
sudo ufw allow 27020/udp