I know GoG and Steam are different companies but I would like to know if it's possible to get it added to my Steam account after buying it here in GoG.

Nope. Anyway, why would anyone want to do that? :-)

I was told it would be easier to share your solutions and compare them with other friends and the community.

It's possible to add it to your Steam library as a non-Steam game. This will allow you to store and upload screenshots on the Steam cloud and give you access to the in-game Steam overlay. Your Steam friends will also see you playing it as a non-Steam game.

However, it will NOT add the Steam version to your library, nor will it track play time, drop cards or offer other features of Steam versions. Beyond using F12 to take screenshots and having the shortcut in your Steam library there is little reason to add it to Steam as a non-Steam game.

As for browsing the community features/guides or contributing to Steam guides, you do not need to own the Steam version. Anyone can contribute.

And as for why anyone would add a non-Steam game to Steam, well Steam users like Steam and belive it is a very nice thing. So there's that. :) It's also nice to have all your game shortcuts in one library rather than scattered about.