After nine seasons with the New England Patriots, NFL star Rob Gronkowski announced on Instagram that he has officially decided to retire from the game. "It all started at 20 years old on stage at the NFL draft when my dream came true," he wrote, "and now here I am about to turn 30 in a few months with a decision I feel is the biggest of my life so far."

The superstar tight end was signed to a six-year, $54 million contract with the New England Patriots. Although he was one of the highest-paid tight ends in the league, the 29-year-old had not touched a dime of his NFL money. As he revealed in his 2015 book, "It's Good to Be Gronk," he lived off his endorsements instead. Gronkowski, who teamed up with Honey Nut Cheerios for their Good Rewards program last year, says that as a veteran on his team, there is one piece of advice he would always gives his rookie teammates about managing their money. "Financially, I just say: Keep it simple," he tells CNBC Make It. Unlike many other professional leagues where athletes' contracts are fully guaranteed, NFL players' are not. That's why Gronkowski says he would always tell his young teammates to "get what you need to live comfortably but don't go crazy with splurging until you feel comfortable in the league."

Rob Gronkowski Tom Szczerbowski | Getty Images