The singer broke the news as she rang in 2015 by taking to Twitter.com to share a mocked up picture of the couple in wedding gear emblazoned with the slogan "Getting Married".
New Found Glory musician Gilbert went on to reveal he proposed on Christmas Day (25Dec14).
Williams was later flooded with messages of congratulations from stars including Taylor Swift and Katy Perry, and she added in a tweet, "Can't even keep up w/ (with) the sweet texts, emails, tweets. Thx (thanks) so much yall! We're so psyched! See yall in 2015 (sic)."
The couple has been together since 2008.