The quirky pop singer was clearly in her element as she joined the platinum-haired designer at the Versace headquarters in Milan, dressed head to toe in her new BFF's designs.
Gaga – real name Stefani Joanne Angelina Germanotta – showed off her new curvier figure in a series of outfits, including a striking orange dress with a revealing bandage upper section and an ensemble featuring a bejewelled top.
The 26-year-old switched her hair colour too – from shiny blonde in the same shade as Donatella's, to brunette.
Gaga and the fashion maven had fun hamming it up for the cameras, blowing kisses from a balcony with the Born This Way's pet pooch Fozzi Bear.
The singer even threw flowers down to crowds of fans who had gathered below.
The pair were heading out to dinner together, much to the designer's delight.
"Looking forward to hosting Lady Gaga for dinner tonight in Milan. She is so inspiring. I cannot wait!" wrote the 57-year-old earlier on Twitter.
The singer has recently been in the headlines for gaining 25lbs, but says she has no plans to diet and is happy with her fuller figure, not least because her other half Taylor Kinney prefers it.
"My boyfriend prefers me curvier," she said.
"When I eat and am healthy and not so worried about my looks. I'm happy.
"Happier then I've ever been. I am not going to go on a psycho-spree because of scrutiny. This is who I am. And I am proud at any size."
She has revealed the reason behind her weight gain is she loves dining at her dad Joseph's new Big Apple Italian eaterie.
"I love eating pasta and I'm a New York Italian girl. That's why I have been staying out of New York.
"My father opened a restaurant. It's so amazing ... it's so freaking delicious, but I'm telling you I gain five pounds every time I go there.
"So my dad wants me to eat at the restaurant, and I'm like, 'I've got to go where I can drink green juice'."