Gio Gonzalez walked into the Oakland Athletics’ clubhouse in August 2008 and immediately began breaking every one of baseball’s unwritten rules, starting with the chapter on How a Rookie Must Behave. In the clubhouse, he committed the unpardonable sin of placing his personal iPod on the communal speaker dock. On the field, he fumed at every borderline ball-strike call, berated himself over walks and threw things in the dugout.

“I sat him down,” said A’s lefty Dallas Braden, himself just a second-year player at the time, “and explained what was going to happen if this kept up — and it wasn’t going to be pretty. I had to give him some tough love and tell him to cool it for awhile.”

By now, Gonzalez, a 26-year-old native of Hialeah, Fla., has matured into a force — a solid clubhouse citizen, a 2011 all-star and one of the best young left-handers in the game. But now, Gonzalez is also long gone from Oakland — having been the cornerstone of a six-player trade in December that brought the Nationals the front-line starting pitcher they craved, and that cost the A’s a pitcher who had become beloved by teammates and coaches alike.

It was one of three major trades made by the A’s this winter, each of which cost them an all-star pitcher — Trevor Cahill (to Arizona), Andrew Bailey (Boston) and Gonzalez.

“It hurts,” Braden said of the Gonzalez deal. “I was his mentor. I was the guy he was always following around. It’s awesome to see him put in the work and get to where he is today. But it’s tough, because I don’t have my little brother to beat up anymore.”

With Gonzalez, unlike other pitchers, the trick was not to coax more out of him. Instead, the trick was dialing him back. Calm down, Gio. Take it easy, Gio. Don’t overthrow your fastball, Gio. Don’t be so hard on yourself, Gio.

“He’s such a perfectionist,” said A’s catcher Kurt Suzuki, “and sometimes he’s too hard on himself. When things don’t go his way, he gets irritated.”

Eventually, the A’s identified the precise location of the line of demarcation that separated what Braden called “good Gio” from “bad Gio.” It was a line Gonzalez needed to approach, giving him the room to be exuberant and fiery on the mound, but across which he could not be allowed to go.

“You can’t put the reins on him and try to make him something he’s not,” Braden said. “If you let Gio be Gio, you’re going to get the guy [the Nationals] just gave $42 million to. Let him be exuberant on the mound.”

Television cameras would frequently catch Braden sitting next to Gonzalez on the dugout bench between innings — sometimes speaking calmly and earnestly, other times more animatedly.

“He needs the excitement and the exuberance and the emotion on his sleeve,” Braden said. “But at the same time, he has to walk a fine line. You cross that line, and now you’re just an act — and it’s a tired act, and no one wants any part of it.”

Gonzalez’s single greatest pitching weapon — a curveball Braden called “otherworldly” — followed a similar trajectory. Gonzalez needed to harness the devastating pitch to where it could be trusted in all sorts of counts, against all sorts of hitters, but not so much so that it was robbed of all its power.

“For most young pitchers, the key is fastball command — because everything works off that,” A’s pitching coach Curt Young said. “But for Gio, it was curveball command. He always had a swing-and-miss curveball, but now he can throw it for strikes when he needs to, or throw it off the corner when he needs to.”

Added Braden: “It’s the best left-handed breaking ball in baseball, period. He makes grown men look absolutely juvenile. And it’s a pitch he knows can throw at any time, whenever, wherever. He could tell a guy, ‘Curveball down the middle, good luck,’ and the guy won’t even be able to get the bat off his shoulder, because he’s bailing out. It’s unfair — because then, he’s going to run 96 [mph] up at your chest.”

Gonzalez’s transformation has been tangible and stunning. After a breakout season in 2010 (15-9, 3.23), he was named an all-star last year in the middle of a 16-12 season with a 3.12 ERA. His ERA dropped each year he spent in Oakland, while his walk rate declined each year.

Asked how Gonzalez will perform in his new environment, Braden offered a bold prediction.

“[Stephen] Strasburg has all the hype in the world,” he said, “but here’s what’s going to happen: You guys who don’t know Gio, you’ll end up more impressed with what he does. You already know what to expect out of Strasburg. But when you see Gio, you’re going to go: ‘[Expletive]. I knew he was good. But [expletive].”