By Nick Selbe The four-time Super Bowl champion has spiked his last football in the NFL. It’s official: Gronk has spiked his last football. Or at least he has as an NFL player. The four-time Super Bowl champion announced his retirement on Tuesday, thanking the Buccaneers with an Instagram post. “I want to thank the whole entire first class Buccaneers organization for an amazing ride, trusting me to come back to play and help build a championship team,” Gronkowski said in the Instagram post. “I will now be going back into my retirement home, walking away from football again with my head held hi…