Sonja's husband, Andre Dubois, posted the sad news Friday afternoon. He said she died "peacefully and beautifully" in their home.

Gaga just posted a photo of Sonja and Andre on Thursday with the message ... "Watching one of my best friends in the world show unfathomable courage, strength, and positivity while battling cancer has really changed my life. Thank you Sonja for inspiring me and all of us everyday with your joy."

Gaga's album 'Joanne' features a bonus track titled "Grigio Girls" ... which is about how she and her girlfriends would get together, pop open a bottle and cry for Sonja.

She also dedicated an acoustic version of "Edge of Glory" to Sonja during her headlining gig at Coachella.

RIP.