Brady took to the comments on Gronkowski's Instagram post announcing his decision, calling Gronk the "G.O.A.T." and showcasing his love for the future Hall of Famer.
Others took to social media to show their support for Gronkowski, like Cowboys running back Ezekiel Elliott and Gronkowski's girlfriend, Camille Kostek.
"Words can't express how proud I am of you," Kostek said. "It has been an absolute thrill watching you perform like no other out on that field. You are the best that has ever played the game if you ask me! I love you with all my heart! I will be your biggest fan in all that you do!"
The tight end won three Super Bowls in his career, helping the Patriots take down the Seahawks in Super Bowl XLIX and breaking plenty of records over the span of his nine-year career.