Today there is a Restock on the Gleam.io website of keys for the Pixel Puzzles: UndeadZ and Suicied Guy Deluxe Edition games with activation in the STEAM store. Gleam.io's website is reliable, I've gotten several free games from them.
To get the key for both games, just click on the link below, then you link your STEAM account with them, and do the TASK that the site asks for.
Addendum: "The game" Pixel Puzzles: UndeadZ comes with 3 games, it plus Pixel Puzzles Traditional Jigsaws and Pixel Puzzles Ultimate. The 3 Counts as a +1 in your STEAM library and only Pixel Puzzles Traditional Jigsaws have no Trading Cards.
Note: Maybe the Keys of the Pixel Puzzles games are gone, but I'm not sure.
Link ( Pixel Puzzles: UndeadZ Gleam.io ): https://gleam.io/6KqbR/grab-a-free-pixel-puzzles-undeadz-steam-keys
Link ( Suicide Guy Delux Edition Gleam.io ): https://gleam.io/4bWSC/suicide-guy-deluxe-ed-giveaway