She revealed the news when she referred to the handsome talent agent, as her "fiancé" while speaking at the Elle Women in Hollywood event.
The revelation came as Gaga was honoured at the star-studded ceremony.
Introduced to the stage by Jennifer Lopez, the Grammy winner thanked her fiancé for his support after delivering an emotional speech about her experience of sexual assault, suffering PTSD and chronic pain.
Explaining her decision to wear a trouser suit, she began: "As a sexual assault survivor by someone in the entertainment industry, as a woman who is still not brave enough to say his name...
"As a woman who lives with chronic pain, as a woman who was conditioned at a very young age to listen to what men told me to do, I decided today I wanted to take the power back. Today I wear the pants."
Lady Gaga - real name Stefani Germanotta - and Christian have been inseparable since going public with their romance in February 2017.
The Bad Romance hitmaker sparked rumours of an engagement way back in November when she was spotted with a sparkling ring on her wedding finger.
Christian is a big time Hollywood agent who represents the likes of Miley Cyrus, Justin Bieber, Harry Styles and Simon Cowell.
The couple have been relatively tight-lipped on their romance but the singer did open up about their romance mid-tour in August 2017.
She gushed to Page Six: "When I went backstage, Christian was by the stairs making sure I was okay. “The best part about being in love with someone is having them there to catch you if you fall.”