I think you may try my tutorial. After adding the game to Galaxy and restarting client check if verify/repair under more -> manage will work for you.

I think goggame-1207664663.info you'll create in main game directory should contains as follow:

{
"gameId" : "1207664663",
"rootGameId" : "1207664663",
"standalone" : true,
"dependencyGameId" : "",
"language" : "English",
"name" : "The Witcher 3: Wild Hunt",
"playTasks" : [
{
"isPrimary" : true,
"type" : "FileTask",
"path" : "bin\\x64\\tw3.exe",
"workingDir" : "bin\x64"
},
{
"name" : "FAQ",
"type" : "FileTask",
"path" : "faq.txt",
"workingDir" : ""
}
]
}

where tw3.exe stands for game executable name. I don't have the game and can't check if this will work..
You may also try downloading standalone installer, for some downloads faster than Galaxy default.

This is exactly the point... so are you saying that there are certain files that come with a GOG *installed* game that allow Galaxy to recognize that game, and without it it doesn't? What if he already started downloading the Witcher 3 and then copied his friends files in that same folder, would Galaxy recognize that part of the game is already finished? No we're not ... this guy just wants to save time downloading his game. What 'installer' are you even talking about? His friend's Steam version isn't an installer, it's the extracted version of the game. Most likely the only thing 'DRM'ed', if that's even the case because CDP certainly wasn't forced to do that, is the exe. If Galaxy somehow manages to recognize all the other files that would save him tons of time.

The only way the above makes sense to me is if Galaxy downloads an installer first and then executes it, rather than install the already extracted game into the apropriate folder, and I'm fairly sure it was doing the latter.

Maybe you did, but you failed at comprehension.

Yay for people not understanding my explanation.

I say no to transfers from gog.com to Steam and no to transfers from Steam to gog.com.

umm... would it not be in gOg's own interest if you can redeem gOg games on Steam? In that case gOg get's the 30% cut (while Valve get 0%) and in addition gOg do not get the cost of providing the bandwidth, while Valve is stuck with that cost... just saying...

Ignore the uninformed "no"s, it's worth a try. Copy the files across however you want, open Galaxy, browse to the game in you library and click on it. Click "more" and there should be an option called something like 'import folder'. It should try and verify the files. Might work, might not.
This isn't what the OP was asking. You didn't understand their question.

That 'extracted' version isn't everything, there are registry entries and bunch of other stuff that are required for game to work. It isn't just one file that you can switch and from Steam it becomes GOG version.

Remember - Steam's copy is copy-protected and while probably 99%+ files are the same in both versions, it's not possible (for user) to make it work that way.

Are you sure? have anyone tested this yet?

He doesn't have to, the question is whether Galaxy can. If Galaxy is just running a checklist of which files it still needs to download and 99% of that checklist is done then all that remains to download is the remaining parts it needs to get it working, including whatever it needs to set up the game.

Except Galaxy verifies and fixes games when you import them, so it should replace the files that are different and make the game work. It's worth a shot anyhow.

I asked but it sank to page 4 in a minute or so. I'm interested to see if it is since it's not a requirement on Steam to have either CEG or the client requirement so they certainly weren't 'forced' to do so, or if they were that'd certainly be interesting information to have.

And begs the question why some games yes and other games no.

OP starts with: "I bought The witcher 3 from Gog"

You start with: "If you want The Witcher 3: Wild Hunt on gog.com, then purchase The Witcher 3: Wild Hunt from gog.com."

So no, you did not read what he said. You read some of it and rushed to posting. He didn't ask that a gog game be redeeamable on Steam or vice-versa. And sorry to burst your bubble but vice-versa, meaning from Steam to GOG, there are high chances of that being possible in the future just how it was possible for Witcher 2. They most likely are waiting for the initial purchase wave to pass.

But it fixes GOG games, it doesn't have an option to 'transform nasty Steam version into neat GOG version'. Yet ;-)
Common sense!

Lets try common sense:

Fact 1: CDPRed have many times stated that they are against DRM.
Fact 2: It is possible to deliver DRM free games via Steam. It is up to the developer/publisher whether they want to make use of DRM or not.

Common sense conclusion: Witcher 3 is DRM'ed on Steam?

@OP:

Go to your friend, see if the Steam version opens without the Steam client, if not go ahead and copy it. You already bought the game on GOG so no harm done. ;)