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My friend owns the game on Steam, but I'd much rather buy the game here on GOG if possible, so I'd love to know if anyone has tried closs-client co-op and had success. Thanks!
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In Drox Operative and Din's Curse there is a direct connection option, if Zombasite have that then yes. You can connect to others with their IP address as long as the game's version match.
Where you buy the game shouldn't have any impact on who you can play with.