Metro: Last Light Redux is being offered for free on GOG.
It is the year 2034. Beneath the ruins of post-apocalyptic Moscow, in the tunnels of the Metro, the remnants of mankind are besieged by deadly threats from outside – and within. Mutants stalk the catacombs beneath the desolate surface, and hunt amidst the poisoned skies above.
Can click through the site to claim or alternatively open the GOG Launcher, click 'Store' then scroll down until your see Metro: Last Light Redux and click claim.