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Like the subject says. I don't see any indication that the GOG version of Clone Carnage has online MP, there's only mention of local splitscreen. Is online only available on Steam? And additionally, are there no options for LAN multiplayer?
Not sure, Clone Carnage doesn't really have an online player roster in general. While steam users can use online play, you can't find a server most the time to play on.

For now, I am under the assumption that there is no online play with GOG due to lack of mention of online multiplayer in product description and many other problems. I expect this game to never get online multiplayer.

GOG isn't popular in the mainstream and a lot of people that use gog share accounts. Also GOG being DRM-FREE makes it hard for game devs like to implement secure online multiplayer.
I can confirm that there is online multiplayer for GOG's DH:CC.

The issue is the same with Steam and Epic, there isn't a large enough player base for it.

If you and a friend have a copy you will be able to find each other.

A recommendation, join the discord for Destroy All Humans and try to schedule playtime with other people.

Feel free to add me to play as well, I will play this for hours.