^This.

If you want achievements and the like this won't be of use to you, but you can launch and play non-Steam games through the Steam client. From the client, click on "Games" and go down to "Add a non-Steam game to my library" and you can still start up the game from the Steam client and play it. So if you just want the convenience of being able to see the game and start it from a single window, you can do that.

A much better downloader that is 8 times quicker. It is way easier to do things with the games in your account. The backups are super easy to use And Drum roll: they know that you can't run 2 anti viruses at the same time .
If you use the Galaxy downloader it is bundled with a separate active antivirus which since windows 95 it is a well known Noob fact you never run more than 1 anti-virus at a time. I am not a fanboy of steam but whoever came up with the wonky Galaxy downloader must be an aspiring programmer with 1 week of training to make such a simple but tragic mistake.

hahaha Wut?

Edit to add: Download speeds are fine for me. And I've never had an anti-virus download through any GOG download. You're full of it.

Unfortunately you get no access to using any features of a migrated game to steam . I tried that & there is Zero benefit besides having another game you can launch through steam.Too bad I was hoping it would load on steam when I reformat but not the case.I could save 2 days loading witcher 3 GOTY.
Why are there countless pages of people that say it takes forever for 70 gigs to download. My computer only takes 17 minutes for 70 gis of info using internet downloader. I am on my second day for witcher 3 GOTY. even GOG support has said it takes that long to download on Galaxy & you really should look on the discussion boards about the antivirus being bundled. Get educated my man before you talk engage your brain. Really check out the discussion boards on GOG Galaxy downloader & you will read exactly what I just said & everyone knows they do bundle a seperate antivirus in the downloader. Check it out & get enlightened

1: My speeds exceed 11MB/s. I cannot vouch for everyone, but it is far more likely that the bottleneck is not on GOG's end, or the user's but rather somewhere in the middle on the route between.
2: After a google search of the forums, nothing turned up about an antivirus. Show me proof of the anti-virus, because I've downloaded hundreds of games, literally through galaxy and never once had any PUP or 3rd party app or program downloaded or installed. So please, enlighten me. Otherwise I'll tell you, you're spreading FUD.

"Everyone knows the Earth is flat." (And by "everyone", I mean "some crank whose posts I just read in a random thread on an unspecified Internet forum, where I probably breezed right by dozens of comments from other users who actually know what they're talking about, calling that person a moron or an obvious liar"; and by "knows", I mean "irrationally believes, despite any direct evidence, and despite numerous reasons why it couldn't be so".)

Oh, you think you're so clever, but it's turtles all the way down, young man.

Shouldn't a thread be locked after it's solved?

Not usually. At least that's not something we teach our clients. Allows for folks who use Google to find the answers to their questions to follow up.

Plus we don't have moderators or even staff presence here.

Ok, support answered that they can't refund (obviously) but they offered me a gog wallet refund, which i won't take since i will definetly play the game now that i downloaded it. Not having it in my steam library will be a little unconvenient, but i'll get over it :D. The thing is that i think i already have too many launchers (steam, battle net, epic games, wargaming, uplay,...) it's starting to get messy, that's why i try to have as much games as i can in the same place, which is the easiest : my steam library.

Anyway, thank you all for your answers !

The thing is with games purchased here, you don't need to use a launcher unless you want to.

Like drmike suggested, "all my games in one place" is a really big reason.

As drmike said, you don't need to use a launcher for GOG games so you can just use a desktop shortcut, or even add it to Steam if you like to launch your games from there.
Threads that disappear without warning are usually auto-deleted by users reporting the OP as spam. When staff do something they usually just lock the thread.