Since Gwen is Roman Catholic, she doesn't want to have a wedding until her first marriage to Bush frontman Gavin Rossdale -- who reportedly cheated on the former No Doubt singer during their 13-year marriage -- has been annulled.
"They had a moment," Rose insisted, before recalling a sweet behind-the-scenes moment she caught on The Voice set.
"We were backstage getting ready and she was getting her makeup done. He stopped and it was like -- so much going on that nobody saw it but me -- but he was standing there and he looked at her like he was so proud. So it's gonna happen."
"He's smitten by her," elaborated the "Team Gwen" member, who will compete inThe Voice finals. "He's going to ask. He is in love with that lady. You could see it all over him."
Engagement and marriage rumors have followed the couple ever since they got serious.
The Voice airs its Season 17 finale next Monday and Tuesday night on NBC.