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I noticed the description page lists the game as single player and it says online multiplayer is not supported. Is this something that will be implemented later or will it stay single player? I feel like it would be incomplete without it since the Steam version has online multiplayer, and it looks like it has some online only features.
The Steam version uses Steamworks for its multiplayer, so this is why its missing on the GOG version. They may be able to implement back using GOG's online systems, but it wouldn't have crossplay to the Steam version.
Yeah, I was thinking that. I'm just wondering if they're planning to add it later on or not. They had to make modifications to remove it for GOG, so they could probably modify it to use GOG's platform. Crossplay on the other hand I don't expect them to implement since it would be a lot more work for them to do. I'd just like the option to play online if I want, since the features look like they could be fun. Without the option to play online, I feel like I'm not getting the full game.
Or even LAN game or internet by specyfying IP.

That way it still would be able to play, even if official server will shut down.

And please bring back DeLorean!

BTW, DeathKarz has no DRM, has no internet restrictions on online-gaming,are more fun IMHO, and are even available on GOG! The only downside is that they don't have DeLorean at all ;-)