Witherspoon lives with her two children -- 8-year-old Ava and 4-year-old Deacon -- from her previous marriage to actor Ryan Phillippe. Witherspoon and Gyllenhaal, who co-starred in the 2007 film "Rendition," have been dating for about a year.
"Marriage is definitely what they are working toward," a source told Us magazine.
"They literally don't want to spend any time away from each other," added a friend of Witherspoon's.
Witherspoon is best known for her roles in "Walk the Line" and "Legally Blonde." Gyllenhaal has starred in the films "The Good Girl" and "Brokeback Mountain."