I want to buy cyberpunk from GOG to directly support the developers however i am concerned about the future online mode. Will steam, GOG, and epic players all be able to play together in the future? if so, I will definitely buy from GOG.

We don't know yet and CDPR probably don't know either yet, but they would be very dumb to split the different stores.

The multiplayer is a small side project right now and they are still figuring stuff out. Before they really start with the multiplayer they'll finish the singleplayer with its free DLCs and expansion(s), just like the The Witcher 3.

OK, this should not be a problem, like GTA V. (OK, other Studio), but However Cyberpunk main design is a singleplayer game.

My concern is more on whether it will be included in the same client as the single player. I hope not.

Watching devs/publishers go after single player mods, mod makers and mod users over the past few years because the multi and single player modes were linked hasn't been a fun time. I'm of the personal view that mods extend the life of games, and I'm rather tired of multiplayer pursuits getting in the way of modding communities when it comes to the single player mode of games.