Rob Gronkowski -- MY DAD'S JACKED ... Check Out These Guns!!!

The JACK DIESEL apple doesn't fall far from the JACK DIESEL tree ... and the Gronkowski family is living breathing proof.

Check out these pics of New England Patriots tight end Rob Gronkowski breakin' out the guns in Vegas this weekend ... right next to his almost-equally swoll father, Gordon!

The whole shredded family hit up the Rehab party at the Hard Rock Hotel to celebrate Rob's 26th birthday ... including Rob's bros Chris, Dan, Glenn and Gordie Jr.

BTW -- the people at Rehab gave Gronk a cake for his birthday. Can you guess what he did with it?