Internet-Security.com UDP port 27020 details

UDP port 27020 is widely used by Valve game servers for spectator relay services.
In Source-engine titles, SourceTV/GOTV listens on UDP 27020 by default (tv_port), e.g., on Counter-Strike: Global Offensive servers for spectators and match recording.
Team Fortress 2 community servers use the same default SourceTV port 27020.
Left 4 Dead and Left 4 Dead 2 servers also use UDP 27020 when SourceTV is enabled.
Garry’s Mod and Insurgency (2014), both Source-based, commonly use UDP 27020 for SourceTV/GOTV.
In older GoldSrc games like Counter-Strike 1.6, HLTV (the predecessor to SourceTV) likewise defaults to UDP 27020.

UDP 27020