How can I get the multiplayer to work through GOG? I want to relive the past and frag by brother over and over again.
A nice step-by-step would be great.
Thanks

1) For Quake 3 Arena Itself --> https://ioquake3.org/help/players-guide/#windowsinstallation

2) For Team Arena --> https://ioquake3.org/help/players-guide/#teamarena

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For Quake 1:

* Quakespasm
* NQuake (Multiplayer Quake 1 Only)

For Quake 2:

* Yamagi Sourceport for Quake 2
* Quake II Starter with Q2Pro Multiplayer Client

Or just direct-ip (/+dedicated server tool optionally I guess still work). Lan (own IP, own Local IP or "connect locahost" for host PC with dedicated tool using) or network. For internet servers you can manually find IP addresses in google (server list sites)

Or emulate direct-ip over lan by GameRanger (optional).

For tet-a-tet play in home with bro - start MP game from in game menu and bro should connect by local router address (192.168....*host PC number at the end*) entering. No 3rd party solutions required.
3rd party solutions required to automated processors online play with random ppl, no need to manually search for servers.