Tom Brady's retirement will mark the end of TWO Hall of Fame careers ... so says Chris Gronkowski, who tells TMZ Sports his bro, Rob Gronkowski, will officially quit when the QB does.
"Whatever Tom has left," Chris says, "I think that's what Rob has left as well."
As for why ... Chris says it's simple -- he just can't fathom Rob sticking around and catching passes from a different style of signal-caller.
"I just can't see him playing with another quarterback," Chris tells us.