Not an app, but if you sign up for Humble Bundle's newsletter, they'll offer a free game to subscribers a few times a year. They're usually Steam keys of well-regarded indie games from various publishers (including Humble Games), with the odd big-budget title thrown in every once in awhile. You'll need a (free) Humble account to take full advantage of this.
In addition to UPlay and GoG, Steam and itch.io sometimes have free games available, aside from the usual free-to-play stuff. The Epic Games Store is still putting out monthly freebies, iirc, though this platform is still considered to be vastly inferior to Steam.
To keep up with free game releases, you might want to check out the blog Indie Game Bundles, which covers bundles and freebies from all the places I mentioned and more.
(Oh, and welcome to the wonderful world of PC gaming!
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