Rob Gronkowski Models Tom Brady's Underwear Brand With Camille Kostek

Tom Brady is supporting Rob Gronkowski ... 's junk -- the future Hall of Fame tight end is modeling Brady Brand underwear while showing off his ripped bod!!

Gronk -- who won 4 Super Bowls playing alongside Tom -- is reconnecting with the G.O.A.T. once again ... showing off some undies from TB12's apparel brand alongside his stunning girlfriend, Camille Kostek.

"Brady to Gronk once again, this time off the field with a @bradybrand underwear pass! I sure scored after that play hehe! Gronk wrote on social media.

In the pic, 33-year-old Gronk and Kostek are wearing all white underwear ... and judging from the pic, Gronk's abs look like he's ready for an NFL comeback!!

Gronk retired for a second time earlier this offseason ... leaving 45-year-old Brady to try to win an unprecedented 8th Super Bowl ring without one of his most trusted targets.

Gronk -- who's retired and unretired before -- said this time, his football playing days are REALLY behind him ... even if Brady comes calling.

"Yeah, I'm done with football," Gronk said. "Love the game. Definitely blessed with all the opportunities the game of football has given me and the relationships I’ve met."