On Saturday, the two were photographed relaxing poolside, with a bikini-clad Witherspoon sipping on a Pina Colada.
Later, she cuddled up to Gyllenhaal during a set by Jenny Lewis. They were also spotted rocking out to Paul McCartney, Morrissey and the Fleet Foxes.
Witherspoon blended in with the crowd in a gray dress, fedora and flip-flops. Gyllenhaal wore a matching gray hoodie.
Us Weekly recently reported that Gyllenhaal threw Witherspoon a 33rd birthday bash March 21 at his Hollywood Hills home.
After toasts from friends (including Robert Downey Jr.), Gyllenhaal paid tribute to his girlfriend of two years.
"Jake said he was so happy they found each other and she meant everything to him," a source tells Us.
Witherspoon - mom to Ava, 9, and Deacon, 5, with ex Ryan Phillippe - is even ready to say "I do" again.
One Witherspoon insider predicts an engagement this year: "All of Reese's friends tell her they think he'd The One." Adds another, "It's not a matter of if, but when. And when Jake asks, she will definitely say yes."