The two met while performing in Dangerous Liaisons on Broadway in April 2008, and began dating a year and a half ago.
Gummer's other stage credits include Mr. Marmalade, Hunting and Gathering, Uncle Vanya, and most recently, School for Lies. She recently starred on the ABC series Off the Map.
Walker starred in Bloody Bloody Andrew Jackson on Broadway last season and also appeared in the Broadway revival of Inherit the Wind with Brian Dennehy and Christopher Plummer. In addition, as previously reported, he will play the title role in the forthcoming film Abraham Lincoln: Vampire Hunter.