What gives? Or, rather, gave?
Tom has never struck out a ton of guys; his strikeouts-per-nine-innings rate held roughly constant over the three year period. He allowed a pretty similar number of hits to opposing batters in both '07 and '08, as well.
If it's not K's or H's, you gotta look at BB's and HR's (trust me, that's just what you gotta do.) So let's do that for Tom:
2007: 3.03 walks per nine, 0.80 home runs per nine innings
2008: 5.98 walks per nine, 1.71 home runs per nine innings
There you have it. In 2008, Tom started walking a ton more guys, and gave up a lot more home runs. So, yeah, that's gonna, like, inflate your ERA and stuff.
What happened in 2009? Actually, Gorgonzola struck more guys out than he ever had before. And while that trend may or not be sustainable, you gotta look at the numbers on walks -- back down to 3.26 per nine -- to really start feeling good.
So what to expect from Gorzo in 2010? Personally, I doubt he'll continue to strike out a batter an inning, just based on every inning he's ever pitched before 2009. However, Gorzo has shown much better control in the past than he did in 2008, so I believe he may sustain that better walk rate into 2010. And from what I've read, anytime a pitcher gives up 1.71 HRs per nine innings, it's more fluky than anything else.
Conclusion? So long as the Cubs trust Gorzelanny enough to give him a shot at the starting rotation (and if they pick Samardzija or Silva over him, God help us all), I think he'll be a pretty decent 4th or 5th option. 4.50 ERA, maybe 80 Ks in 120 innings, and hopefully closer to 40 rather than 80 walks.