After the Academy of Country Music Awards, Blake Shelton performed a surprise pop-up concert and his girlfriend couldn’t help but get in on the act.

On Sunday night, Shelton tweeted that he was going to perform a free show at the House of Blues in Vegas after the ACMs and was bringing along some “friends.”

Hey #Vegas!! My friends and I are taking over the @HOBLasVegas TONIGHT and guess what?! IT’S FREE! Yup...that's right y'all! First come/first served for admission. Doors at 10pm. 18+ pic.twitter.com/WApJNG8hUK

— Blake Shelton (@blakeshelton) April 16, 2018

And he certainly delivered with Kelly Clarkson and Stefani joining him.

His lady love took the stage around 1:30 a.m. Monday and performing several songs, including “Hollaback Girl” and “Hella Good.”

“The crowd was going crazy for Gwen,” a concertgoer tells PEOPLE.

Stefani wore jeans, boots and a green jacket while letting her blonde hair loose for the night as she led the crowd into a soaring rendition of “Hollaback Girl.”

But boyfriend Shelton had a special request, walking onto the stage – much to the excitement of the crowd – and asked her to sing “Hella Good”.

After a bit of a convincing, the songstress relented – if he agreed to sing it with her.

Gwen Stefani!! https://t.co/zxaQo3O7OR

— SDGalGina (@SDGalGina) April 16, 2018

“I’m only doing it if you do it with me,” she said.

Toward the crowd, Stefani said, “So, I know it’s weird that I’m up here… but we all have one thing in common and it’s that we love Blake Shelton very much.”

During the song, Shelton, red Solo cup in hand, helped Stefani sing the chorus. He also jokingly danced for his lady love.

The couple recently opened up in separate interviews about their relationship, revealing both had considered marrying the other.

Shelton, 41, admitted to CMT’s Cody Alan that he has marriage on his mind.

“I think about it. I don’t think it’s anytime soon or anything,” Shelton said when asked about tying the knot. “The more time that goes by, I guess, the closer you’d be getting to that.”

Stefani, 48, revealed she also had wedding bells on her mind during an appearance on The Ellen DeGeneres Show last week.

• Want to keep up with the latest from PEOPLE? Sign up for our daily newsletter to get our best stories of the day delivered straight to your inbox.

DeGeneres asked if rumors that the two were planning on getting married soon were true, a question which the singer skirted around by asking DeGeneres for advice.

“I love being married,” the host said. “I think you should get married.”

Stefani said her three sons — Kingston, 11, Zuma Nesta Rock, 9, and Apollo Bowie Flynn, 4, with ex-husband Gavin Rossdale — “love” Shelton.

DeGeneres dropped the topic but told her to keep it in mind.

“I do,” she replied. “I think about it all the time.”