is there any way to connect the game to steam?

Of course not. But, if you want to play the game from Steam, you can always "Add a Non-Steam Game".

it should be, on other platforms you always get cd-keys to use it somewhere else..

It shouldn't be. When you buy a game in GOG, you get a license. When you buy a game in Steam, you are paying for some kind of permission to play the game, which can be revoked at any time, and they don't even have to warn you about it.

True! But I think it'd be more accurately to say Steam gives you license (which can be revoked as you correctly noticed), but with GOG you own the game, it's not a license, so nobody can ever revoke your right to own it.

If you bought the game from GOG, you don't get a key or anything. You just get the game, DRM-Free. You own it.

If you buy retail you get a DRM-Free disk, and I believe a GOG activation code as well. [not 100% about the GOG code]

If you buy from Steam you get a Steam license to play the game. You do not own it.

GOG is by far the best place to buy TW3. I myself own a GOG copy and will be buying a retail copy as well when the price drops so I can have the disk, box, map etc... I've done this with TW1 and TW2 as well. I support CDPR fully and have no issues with owning 2 copies of each game.

Edit: Sorry, to answer your question. NO there is no way to link a GOG/Retail version directly to your Steam account. The only way to play TW3 with full Steam support is to buy a Steam license. As others have said, if you want to launch TW3 through Steam you can add the exe as a 'non Steam game'. Being able to link the 2 would mean using DRM, which CDPR/GOG are against. Galaxy does the same exact thing Steam does without DRM, it will count your playtime, track achievements etc...

That is funny.
http://www.gog.com/support/policies/gog_user_agreement
2.1 We give you and other GOG users the personal right (known legally as a 'licence') to use GOG.com to download and/or stream (depending on the content) and use GOG content and other GOG services. This licence is for your personal use. We can stop or suspend this licence in some situations, which are explained later on.
15. Termination
They can be close your account, and you cannot download the game.

But because the game is DRM-Free, you can keep downloaded files and play it.

On Steam, the publishers not required the put DRM on their games. There are games you can play the with their "exe" without Steam. If the CD Project RED does not put Steam DRM on Steam version, even Steam revoke your license, you can play it if you have files. The situation same with GOG.

The only difference between GOG and Steam. GOG guarantees the games are DRM-Free, but Steam does not. But this not means there is no DRM-Free games on Steam.

Legally it might be called "licence", not gonna argue on this. But essentially...

If one was stupid enough to get himself blocked and didn't get backup installers, well, it his own problem. I always download the offline installer right after the purchase. That way I know I'll be able to install the game any time anywhere even without internet connection.

With Steam, if you get blocked, you may play some (not all) games as you said, but you'll never be able to install the game. On new computers you cannot install games even if everything fine with your account, you will need internet connection.

These situations are not the same.

On Steam, after downloading the game, if the game has not DRM, and you back up the downloaded folder and corresponding .acf file, you can install it. If there is required dependency or registry key, all is written in here. I know it is not a good way, but you can.

The point I am not agree with you is that you own license on Steam, but you own the game on GOG.
There is always licenses on digital commodities. On GOG, if you own the game, it should be legal giving the game to your friends or someone else. But it is not legal.
DRM-Free does not means you own the game. It means "there is some agreement between you and me, but I will not bother you with terrible DRM applications to enforce agreement".

I wish all publisher uses the both. But at this Steam provide more features with either DRM-Free, or not-so-much-terrible Steam DRM, or terrible-drms-which-dont-care-customers.

It's totally wrong. GOG stuff has once made a statement in one of the "piracy" threads that they view games on their site the same way as a dvd or a book. You can lend it to a friend, but don't play this game at the same time, like with a dvd or a book you both wouldn't be able to use it simultaneously. Unfortunately I cannot find the link to the post. Hope someone else can help with the link.

Steam have same feature if the publisher is not disabled, Family Sharing.
However, I cannot see my friend's Witcher 3. I think CD Project is disabled this feature.It is a bad move for a company which supports DRM-Free.

I think CD Project should give Steam key to people who purchased the W3 from GOG. And vice versa.

PS: Steam keys doesn't cost the publisher.

It's not the same.

If you mean Galaxy, it's been released in beta a couple of weeks ago. They couldn't disable something that didn't exist.

They are competitors, why would they do this?

Maybe it's not that obvious but any digital product that you can multiply infinitely cost nothing.

1- Yes, it is true they are not exactly same. But for most part they works same. You can lend your library to a friend while you are not playing.

2- I mean Steam Family Sharing. I cannot see my friend's Witcher 3 in my library (I can see his other games). Probably CDPR disabled this feature.

3- Yes they are competitors, but Witcher 3 already is being sold on Steam. Maybe I want to play it on Steam but I want also all money to go CD Project RED.

4- The reason why I mentioned the keys are free, maybe you say that why do CD Project give you Steam key which they should give money to Steam to generate it.

You can always add non-Steam game to Steam. I've read in the other thread someone claimed he could do it and even use Steam features to get FPS during gameplay. Not sure it was true as haven't followed recent Steam client changes.

I don't like adding non-steam games. I want cloud save, achievements, playtime in my profile etc.