And his predictable attachment to the number 69 was also evident during his days at Woodland Hills High School in suburban Pittsburgh.
"So we're playing a home game and we have 68 points - now it's not a blowout, but it's not a particularly close game either," Gronkowski's high school coach, Chuck Swierski, recalled to WPRI TV channel.
"So he [Gronkowski] gets fouled and goes to the free throw line. He hits the first one to get us to 69 and the place goes nuts, because he's playing into it.
"On his next free throw, he purposely chucks the ball off the backboard and misses, so the score would stay at 69 and then runs back on defence pointing at the scoreboard."
Gronkowski scored the 69th touchdown of his NFL career in October and celebrated his favourite-number achievement with his family, mother included.
"The first thing yesterday - we sat down, I saw her - she came to my hotel with everyone," Gronkowski said. "We sat down and she goes, 'Rob, I'm so proud of you when you score your 69th touchdown'."
Girlfriend Camille Kostek also confirmed that Gronkowski has been saying "69" since he was a toddler.
"Rob told me he's been saying '69' since he was a little kid and I thought that was a total exaggeration," the blond beauty explained Tuesday, during an appearance on the "Barstool Rundown" sports blog.
"He goes, 'Ask my mum'. I said, 'Fine, I'll ask your mum'.
"His mom came over, I asked his mum and she said he's been saying '69' since he was three years old. This isn't a new thing - this has been going on and on."
Gronkowski will be sidelined by injury for Super Bowl LI on Sunday, cheering on, as Tom Brady looks to capture a record-setting fifth Lombardi Trophy.
