The name Bohemia Interactive may not ring that many bells, but it is the studio behind the ARMA series. The devs now have four titles on GOG, which mean that they are DRM Free. They are also very heavily discounted.
To start off are two entries into the aforementioned Arma series. These are ARMA: Gold Edition and ARMA: Cold War Assault. The latter was also known as Operation Flashpoint: Cold War Crisis before getting renamed in 2011. These are tactical shooter that are famed for being very realistic, so they play very differently from your usual first- and third-person shooters.
Following the two shooters are two real-time strategy entries, Original War and UFO: Afterlight. They’re not your typical RTS-type titles either, with the former playing more like Warhammer 40000: Dawn of War 2 and the latter plays more like an X-COM title.
All four titles are far from new. In fact, the newest of them were released all the way back in 2007. But add that to the fact that they all currently have an 80% discount to them, this means you can get them all for slightly over US$5 (~RM21). These discounts will be in effect until 17 July.