When your publicist ain't talkin', go to Gam-Gam!

Justin Timberlake's grandmother, Sadie Bomar, has confirmed that JT and on-again, off-again love Jessica Biel are officially engaged.

Bomar told the website Gossip Cop that Timberlake "called me on Christmas Day and told me he's engaged," and explained that the "SexyBack" singer asked her not to divulge the news to anybody "until after the holidays," adding that "the family loves Jessie. She's a very sweet girl."