> The Quake engine was also released under the GPL, and went on to be highly influential, serving as the basis for dozens of games, including time-honored favorites such as the Half Life series. Large swaths of the gaming canon were made possible thanks to the generous contributions of free software game publishers.

This seems misleading. Half-Life was released in 1998, but the Quake engine's source code wasn't released under the GPL until 1999, so presumably Valve had a non-Free Software license to use the code.