Bohemia Interactive has issued four games from its back catalogue on GOG: all are DRM free, and all are heavily discounted. They include the original Arma, Arma: Cold War Assault, and two strategy games in the form of UFO: Afterlight and Original War.
It's worth noting that Arma: Cold War Assault (opens in new tab) is probably better known as Operation Flashpoint, which was published by Codemasters back in 2001. This 'reissue' bundles the original Operation Flashpoint with its 2002 expansion, Operation Flashpoint: Resistance. It's currently on sale for a measly 99 cents (AU$1.50).
Elsewhere, Arma: Gold Edition (opens in new tab) bundles the core game with its Queen's Gambit expansion. It's technically the second Arma game—it released in 2007, six years after the retconned Cold War Assault—and it really upped the ante in terms of scale: it has a 400 square kilometre map and can host matches of over 100 players. It's currently $1.99 (AU$2.89).
There's also the 2007 Mars strategy game UFO: Afterlight (opens in new tab) for $1.39 (AU$1.99), and cold war RTS Original War (opens in new tab) for 99 cents (AU$1.50). All are on sale until July 17.