SWAT 4 Gold Edition has been re-released DRM-free exclusively on GOG.com. It includes both the base game and its expansion, SWAT 4: The Stetchkov Syndicate, and costs £7.99.
This release is notable because Sierra's 2005 cult-classic tactical shooter was previously nowhere to be found. GOG contacted the original development team behind SWAT and put up an Q&A with creators Paul Hellquist and Bill Gardner and it's worth a read.
SWAT 4 was developed by Irrational, which would later go on to make BioShock. In 2013, Will Porter penned a SWAT 4 retrospective for Eurogamer, calling the game "a brilliant example of the hard-edged squad commands that this console generation's lean to the mainstream has gone to great pains to rub away".