Go to the page of The Witcher 3 in GOG Galaxy and click the More button (next to the big Play button). Select "Manage Installation"->"Configure". In the new configuration window, click the "Features" tab.

If the checkbox of "Launch parameters [ ] Custom executables / arguments" is not checked, click it.

Scroll down until you see next to "My Label" a "The Witcher 3 - Wild Hunt (DX11)", and click the radio button next to "Default executable" right below it.

On a side note (don't read this, go play the game instead), I know the above is very obvious, but I am still laughing that I almost deleted the game because it was intolerably slow and choppy. I am in the middle of a complicated hardware upgrade, and although I do have The Witcher 3 for Xbox, I decided to download my second copy of the game for PC and finally play it. Then I forgot about it, because I was busy checking every other game I own with my new graphics card, while I was making glorious plans to upgrade the CPU next. Didn't even notice that in the meanwhile, the Witcher 3's new massive update has changed the executable to the DX12 one, and when I finally decided to launch it, I started pulling my hair trying every possible setting down to 720p to no avail. Thankfully, the DX11 executable now runs everything on high 1080p even with my old CPU, whereas with my old video card, the game barely moved. Because of that sneaky update, I was actually on the verge of giving up and waiting a month or two until I've finished upgrading the CPU too--downloading another 50 gigs.

End of story, guys. And thank you, Red, for this wonderful game. I just played a whole adequate hour of it, and it's hard to put it down. I can't believe I've postponed it for so long.