The drama, based on a book by John Heilemann and Mark Halperin, focuses on John McCain's 2008 presidential campaign.
Ed Harris has already signed up for the role of McCain, while Julianne Moore will star as his running mate Sarah Palin.
Deadline reports that Paulson has now agreed to play McCain's senior campaign adviser Nicolle Wallace. The actress has previously starred in shows such as Deadwood and Studio 60 On The Sunset Strip.
MacNicol, who has recently been recurring as Dr Stark in Grey's Anatomy, will join the Game Change cast as McCain's campaign manager Rick Davis. Meanwhile, Gaston is expected to play Levi Johnston, the boyfriend of Palin's daughter Bristol (Melissa Farman).
Game Change will also star Woody Harrelson, Ron Livingston, Larry Sullivan and Jamey Sheridan.