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The store page lists multiplayer as being LAN only, this is incorrect, if you read the game's documentation, you'll find you can simply host an Internet game from within the game, then have the person you want to play with add;
+connect (host ipv4):(chosen port)
to the Target line of the shortcut that launches the game.

So to connect to someone, I may change the target to;
"C:\Program Files (x86)\GoG\Codename - Outbreak\OutBreak.exe" +CONNECT 255.255.255.225:42069
then, when I launch it, assuming the person at that IP has an online game running, I will connect to them.

I figured it was worth making a thread about this because, despite it being in the documentation you can do this, the store page says it's LAN only and there's even a Steam group dedicated to playing the game through false LANs like Evolve and such.
Sadly it still does not work.