“Gwyneth had a casual celebration during the day that Chris was a part of,” a source tells PEOPLE. “Many gifts arrived at the house, including flowers and art work.”
The festivities continued into the evening as a parade of celebs including Kate Hudson and fiancé Matt Bellamy, Tobey Maguire and wife Jennifer Meyer Maguire, Nicole Richie and Maya Rudolph, joined Paltrow (and Martin) for cocktails at the house.
“It seems they are definitely getting more serious,” says a source of the new couple. “Whenever Chris is not spending time with Apple and Moses at his Malibu, [California], house, Jen is there.
Ever since the duo returned from Las Vegas on Sept. 20, sources say Lawrence, 24, and Martin, 37, have been inseparable.
“Chris and Jen always time it so that after Gwyneth picks up the kids (Apple 10, and Moses, 8), Jen will arrive in Malibu minutes after,” adds the source.
As for Paltrow, who also celebrated her birthday last week with a party thrown by friend Ryan Murphy, her rumored new boyfriend, Glee co-creator Brad Falchuk, 43, was not spotted at the festivities at all on Saturday.