Gilbert, 39, and Perry, 48, began dating in 2011 and got engaged during a picnic last April.
"I'm sitting there listening and I'm kind of daydreaming and I'm thinking like, 'If I ever proposed to her this would be an amazing way to do it,'" Gilbert said on The Talk after Perry popped the question.
Perry had organized for a group of musicians to serenade the couple with their song, The Cure's "Love Song," during the picnic. She then popped the question, using T-shirts to spell out "Will you marry me?" The couple's mothers and a few close friends were watching from nearby. "It's the most amazing proposal ever," Gilbert added.
Gilbert has two children, Levi, 9, and Sawyer, 6, with her ex, TV producer Allison Adler.