"It was great working out," a slimmed-down Gronk told us at the $50k Charity Challenge Basketball Game at Pauley Pavilion. 

"Tom needs someone to throw to so, you know, he calls Mr. Reliable Robbie G the one and only!"

Rob ran routes and caught passes from TB12 at the field at UCLA, where Tom usually works out while in L.A. for the offseason. Gronk is in town for the charity event, which raises money for Teen Cancer America. He's also expected to attend the ESPYs on Wednesday. 

Of course, the workout with Tom begs the question ... will Gronk come out of retirement for one last ride with his Hall of Fame quarterback?! 

It's possible ... while he admits he's enjoying retirement so far, Gronk told Rich Eisen last week that his attitude might change once the season starts.

"I can’t really say how I’m going to feel about it when the games start rolling around and everything," Gronk said.