The Gameplayers Series was created by Richard Berg as an accessible and fast-playing game system driven by activation markers. It was first and foremost the common game mechanics based on activation markers and not a common theme which defined the series. So at some point the series covered the American Civil War, World War II and the Napoleonic Wars. In the meantime the ACW and Napoleonic games have branched out into their own game series and are therefore not considered part of the Gameplayers Series anymore.
See also the Glory and Triumph & Glory series.