According to the New York Post newspaper, he chatted to a number of women, telling one, "I feel pretty good."
Before attending the bash on Saturday, Scottish actor Gerard was spotted jogging along Sunset Boulevard in Hollywood with his personal trainer, smiling and waving at fans.
He also enjoyed a solo lunch in Malibu after his release on Friday, where he was spotted with only a book for company.
After his release from the treatment centre, his representative, Rupert Fowler, said: "Gerard has completed a successful course of treatment and has returned home in good health."
Gerard had entered the facility after realising he was becoming dependent on prescription drugs.
His problems were allegedly triggered by the physical demands of shooting '300' in 2006 when he started taking painkillers to cope with numerous injuries he got while shooting the action film.
The problem got worse when the 42-year-old actor was badly hurt in a surfing accident when filming 'Of Men and Mavericks' - which tells the true life story of late surfing icon Jay Moriarty - in December.
Gerard got caught in a 12ft swell at the infamous big-wave Northern Californian surfing spot Maverick's.
After being pulled from the water onto a jet ski, he was taken to shore and driven to Stanford Medical Centre where he was kept under observation for one night.