I've only used Steam and Epic for my whole life, and never bought any games from GOG before.
Are there any advantages or exclusive items I can get by buying this game on GOG?
I'm really torn between the two platforms. (There doesn't seem to be any exclusive rewards on Epic)
Can I get some information?

I don't know what is available or considered "rewards" on the other platforms (all I know about Steam and Epic is that you don't own your games there, you only rent them), but here are the list of Cyberpunk 2077 rewards I know of:
https://www.cyberpunk.net/en/myrewards

All of which are available through a simple mod called Cyber Engine Tweaks.

If you're buying games here on GoG, please don't use GoG Galaxy. It's a piece of trash software, and downloading your games and running them locally is much, MUCH better!

Possibly, but while the game still runs locally, Galaxy sends tracking information while running. Which it has to, to track achievements and whatnot. Personally, I don't care about achievements, but I do care about not being tracked when playing games. And Galaxy serves no purpose that I can see, other than sending tracking and statistics information, so I don't understand why Galaxy exists at all.

Interesting point about Galaxy using the same setup files though. There was a long-running issue with installing, at the time, the latest iteration of the game. Turned out some of the offline files were corrupt, and if Galaxy used the same files, obviously Galaxy couldn't fix it either.