I bought The witcher 3 from Gog, but, my download speed is sucks only 50KBps, and it may take weeks to finish the download for 27GB.
I have a friend who bought The witcher 3 in steam, So I want to copy his game client to my Gog.
Is it possible guys? thank you

It's not.

it's hard to say, I imagine most files would be similar. I've seen people say they imported their already installed and *modded* games on Galaxy and Galaxy overwrote their mods with default files. Seems that's exactly what you'd want to happen.

So, maybe?

So sad, I must download it myself then... :(
Thank you Zeffyr! hmmm, I think I will search for that, Thanks for you info!

I think it it is impossible.

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

I hope very much that gog.com does not allow you to copy the Steam version of The Witcher 3: Wild Hunt to gog.com.

Steam is gog.com's competitor.

Most yes, yet it's not enough.

You can ask your friend to download the installers for you. If you have an external disk, you can give it to your friend, login to your GOG account and download the installers to the disk. Then all you would have to do is run the setup on your computer.

Did you try to download through Galaxy ? Because everyone reported that to be very slow. I didn't even get 50KBs when trying that. So I switched to downloading with the GOG Downloader and got over 10MBs. He did buy it on gog. Read it again ;)

I read what he said.

Re-read what I said in my comment. I gave a explanation why to not allow video games purchased from gog.com to be redeemable to Steam or video games purchased from Steam to be redeemable to gog.com.

I really don't understand what you're getting at (mostly because you're not explaining anything). Are you implying that Galaxy redownloads the entire game? Because if it only downloads the files that are "missing" because the Steam ones are different from the GOG ones then at the very least he cuts down on a large part of the download and thus the time it takes before he gets to play, which is exactly what he's after.

As Aningan stated, many people are finding downloading through Galaxy is relatively slow at the moment. You may get much faster speeds by downloading the game installer. To do this click on Witcher 3 in your library, click on 'More', select 'Backup & Extras', and then click on the game files to download them.

There are some files which allow the Galaxy to recognize that these files are TW3 and can be connected with it, that Steam's version doesn't require.

If we're talking about DRM-freeness, Steam's files contain copy-protection, so you can not run it on another computer without logging into account.

So,if you copy just Steam's installer, you'll install Steam version. GOG's installer contains a copy which can be ran anywhere, like on OP's computer.

@Winstonhusky - can't you ask your friend to download the GOG version for you?

Re-read what OP said. He doesn't want to redeem a Steam game on GOG (or a GOG game on Steam). He just wants to know if he can copy the game files from a friend's Steam version to avoid downloading his GOG version (because his internet is slow and it would take him weeks).

Have you tried the optional ways, ie. downloading the TW3 installer files, either with the direct web links, or with the old GOG Downloader client?

I just downloaded TW3 installer files blazingly fast at my workplace (using the GOG Downloader), I think it came down at around 13 Mbytes/sec. It was downloaded during my lunch.