Ryan Gosling surprised Greta Gerwig with a Barbie flash mob for her birthday

The actor's tribute was attended by a bevy of Barbies and Kens who helped celebrate the filmmaker's 40th birthday as a Warner Bros. title continued to run at the box office.

Ryan Gosling gave Barbie director Greta Gerwig an unforgettable birthday bash as the Warner Bros. film continued to dominate the box office.

The Instagram account of the official Barbie movie posted a video of Gerwig freaking out when she walked into the room and looked like a Pilates class as a flash mob of Barbies and Kens in honor of the filmmaker, who turned 40 on Friday. Gosling stars as Ken opposite Margot Robbie's Barbie in the runaway hit that topped the domestic box office in three weekends and crossed $1 billion worldwide on Sunday, the first live-action film by a female director to reach that milestone.

"As Kens know…. Sometimes the only way to express your feelings is through song and dance!" the caption read. The news quoted Gosling's character as saying, "Ken Ryan sent these special Barbies with Kens to Greta's birthday party for start off and everything feels good!!"

This group of Kens perform the dance routine Gosling sings in the movie, "I'm Just Ken." Then a bunch of barbies entered the room and they all joined in another song from Dua Lipa's Dance Night Away.

"That's cute!" a delighted Gerwig exclaims at the end. He then asks who sent it and is told that Gosling came up with the idea. This book has BookAFlashMob.com whose execs help manage it.