The future Hall of Fame tight end says he's gained back the 20-or-so pounds he lost after his Patriots retirement ... joking he's "four more protein shakes away from being at my playing weight."

Rob quit the game back in March 2019 ... and almost immediately dropped down to around 240 pounds, causing everyone to wonder if he was really done with the game for good.

"It felt good just to drop that weight," Gronk told CBS Sports HQ on Wednesday. "You know, ease up my joints, let my body free. I kind of felt like that weight was bloated weight, too. So, it felt good to get rid of it."

But, after agreeing to reunite with Tom Brady last month ... he's already bulked back up, saying he only needs a couple more lbs to be NFL-ready.

"I'm about 257 pounds right now," Rob says ... "I'm excited about that. I'll definitely be at my weight when it's time to go. I'll be at 260 to 265 when it's time to roll."

As for how the weight changes will affect Rob -- who says he hadn't previously been under 260 since his senior of HS -- he's confident the cutting and bulking won't spell disaster in a return to the gridiron.

"It's time to build back upon it," Gronk said. "Put lean muscle on my body and get ready for the NFL season like I never have before."

Music to TB12's ears!!!