"Well, I already knew [she was joining], but I couldn't say nothing," Gibson, who plays Roman Pearce in the movie, told ET's Katie Krause. "I had to [keep it a secret]."
"I feel like her coming on board -- girl power," he continued. "What we pride [ourselves in] in The Fast and Furious [franchise] is a very diversified cast that's translating domestically and internationally in a major way."
"When you look at the movie poster and you look to the left, you look to the right, I'm like, 'Fellas, we need some more women in here, man. We got to change this thing up a bit,'" he further explained. "We have bits and pieces of power women in it, but we needed to bring it up a notch. [We were] looking into the Ronda Rouseys, Michelle Rodriguez has been holding it down since day one, but we got Jordana Brewster and Charlize, we're really stepping it up."
"There's so many powerful women behind the scenes, now we got to get them on the scene," Gibson suggested. "So, Charlize, from Roman Pearce himself -- and girl power herself [my daughter] Shayla Gibson -- welcome to the family!"
Rumors that Theron would be joining the Fast 8 cast as a villain were circulating for months, and the official Facebook page for the action-packed film franchise finally announced that it was indeed happening on Thursday.
"Our crew will face its greatest adversary ever in our next film as we welcome Charlize Theron to the Fast family. 4.14.17. #F8," the post read.
Our crew will face its greatest adversary ever in our next film as we welcome Charlize Theron to the Fast family. 4.14.17. #F8
After chatting with ET, Gibson took to Instagram to share a snap of himself hugging his newly announced co-star.
"The best hugs........ Family business comes with family secrets I've had to keep my lips sealed about Charlize..... Of all things were excited about more #GIRLPOWER!!!!!!!!!!" he captioned it. "Welcome to the #FastFamily you're most welcome here!!!"
ET also chatted with Gibson's 8-year-old daughter, Shayla, at WE Day, who is no stranger to attending red carpets with her famous daddy.
"Well, there's a lot of people [here], but I'm most excited to see Taylor Swift and Selena Gomez," she said. Although Swift did not attend the event, Gomez did, and talked to ET on the carpet.