Does in-game overlay works for you ? I've tried in Witcher 3 and nothing happens.

I can confirm that it works for Day of the Tentacle and Shadowrun: Dragonfall.

I just tried Witcher 3, and it worked fine for me... are you on preview update 1.2?

Yes of course. It's shift + tab right ?

Yes... if you can't get it to work, post the issue on http://mantis.gog.com/ with your logs. The new version generates logs for the overlay, maybe GOG can investigate the issue.

All the games on <span class="bold">this list</span> should support the overlay and the screenshot options. I think others not on the list support it, too, since I've seen a screenshot of Mount & Blade Warband taken with GOG Galaxy as well.