With the Brewers
Yovani Gallardo was something of a divisive player when he was with the Brewers. A lot of people seemed to think he wasn't that good. I think he was a victim of his own hype. A lot of people expected him to be another ace to partner with Ben Sheets and lead the Brewers to their World Series victory. He wasn't ever that good. But he was pretty good.
Gallardo spent parts of 8 seasons in a Brewers uniform. In those 8 seasons he threw 1289.1 innings and accrued a 3.69 ERA and 3.71 FIP. He owned a 22.4 K%, 8.7 BB%, .242 BAA, and 1.30 WHIP. By FanGraphs' metric he was worth 20.7 WAR. That places him 26th over that time span just below Roy Oswalt and just above Matt Garza.
FanGraphs also estimates he value in dollars to have been $137.5 million during that 8 season stretch. In actual dollars the Brewers only paid Yovani Gallardo $38,318,000. So in that sense Yovani Gallardo provided the Brewers with nearly $100 million in surplus value. On top of that they were able to trade him to the Rangers for Corey Knebel, Luis Sardinas, and Marcos Diplan.
Yovani Gallardo was never the ace people hoped for but he was still a quality mid-rotation starter. He was a home grown pitcher who work for or near league minimum for most of his career with the Brewers. And because of that he provided them with immense surplus value.
After the Brewers
I remember hearing a lot of people talking about how the Brewers must be terrible at coaching pitchers because when they leave and go elsewhere they perform better. Yovani Gallardo's first half was used as an example of this. He had a 2.76 ERA. But that doesn't tell the whole story and it's largely a function of his ridiculous June.
He had a 0.54 ERA in June. But in April he had a 4.05 ERA and in 3.75 ERA. His second half--4.69 ERA--was more sporadic. In July he had a 5.46 ERA. August a 1.98 ERA. And in Sept/Oct a 4.85 ERA. In total on the season he accrued a 3.42 ERA, 4.00 FIP, and was worth 2.5 fWAR.
Those numbers were pretty similar to his last season with the Brewers when he owned a 3.51 ERA, 3.94 FIP, and was worth 2.0 fWAR. His peripherals were actually better: 17.9 K%, 6.6 BB%, .257 BAA, 1.29 WHIP with the Brewers vs 15.3 K%, 8.6 BB%, .267 BAA, and 1.42 WHIP with the Rangers.
Overall he was still a decent starter for the Rangers, he just wasn't any better than he ever was with the Brewers. If anything he was more or less the same pitcher for both teams. He's still not that ace people hoped for and he's not quite as good as he once was. But he's still at least an average starter that you can count on for 30+ starts a season.
Free Agent Outlook
Yovani Gallardo enters his first free agency at the age of 29--30 on Opening Day. He's been worth at least 2 fWAR in every full season he's pitched. He's probably a 3/4 SP at best and more likely a straight 4. In a vacuum I'd suggest he'd sign for something very similar to the Matt Garza contract. He actually would probably do better since Gallardo doesn't have the injury history that Garza did.
However Gallardo doesn't get to negotiate his contract in a vacuum. Not only is this offseason full of solid free agent starting options, the Rangers extended the qualifying offer to Gallardo. If he turns it down--which I believe he will--the team that signs him will have to forfeit their highest draft pick. Those two facts will work together to suppress Gallardo's earning potential.
I think he will get a 4 year contract at close to $13 million annual average value. Options are all the craze these days so I'll guess he gets a team option tacked on there with a reasonable buyout round $2 million. That's a similar contract to the ones Ricky Nolasco and Matt Garza got and I think Gallardo is similarly talented.
Statistics courtesy of FanGraphs