That's Rob Gronkowski being brutally honest about his 9 years with the New England Patriots -- admitting it was tough, but worth it.

"I'm very thankful for the 9 years I had with the New England Patriots," Gronk said during a call with reporters on Wednesday ... "I've learned so much under that organization."

Gronk shouted out both Bill Belichick and owner Robert Kraft -- saying he will ALWAYS have respect for those guys for drafting him and growing him into the NFL champion he is today.

But he was honest about the experience in New England -- saying, "I'm not going to sit here and say its an easy organization to play for."

"I've also never played for another organization. I haven't even been down to Tampa yet."

"I know that the NE Patriots, hands down, it is not the easiest place to play but it gets you right. It gets you mentally right, it gets you physically right ... but I'm definitely going to take what I've learned ... and apply it to my daily life, big time."

Gronk didn't get into details about why it was a tough place to play but it seems obvious -- it was an all work, no play kind of a job.

During Wednesday's call, Gronk also explained how influential Tom Brady has been in his decision to come back to the NFL.

"Tom is like the appetizer of the whole meal. He got me hooked when he went down to Tampa."

"Playing with Tom is special," Gronk said ... "We have a great chemistry out there and every time we get together, its just like the old days. It doesnt matter if we take a month off or 6 months apart, we just go out there and we're throwing the ball just like its a normal practice."

After Tom selected Tampa Bay, Gronk says he started looking into the Bucs roster and was impressed with what he saw.

"I saw what was down there. Just the opportunity to go down there and play with that type of offense. I mean they got some special wide receivers out there, they got other great tight end and let me tell you, I love working with tight ends too so I'm definitely looking forward to that, working with [OJ Howard and Cameron Brate]. That's definitely going to be a special opportunity. Just blessed to be in this situation."

Gronk says his decision to retire from football in 2019 was motivated by real injury concerns -- saying his body felt beat up and he needed a break.

"I was in a very painful situation. Even my very last game, I couldn't for four weeks. I couldn't walk that well after the last Super Bowl game we won from my quad contusion."

Rob continued ... "That's what I did the last year. I took care of myself. I let my body heal, I let my body rest, I let my body get the treatments that it needed. I feel like I broke up all the scar tissue that's in my body. Got rid the inflammation in my body that was holding me back my last year, I feel like. It feels good."

Gronk also admits, "I had no fire last year" -- but as time rolled by, "my body started feeling good."