According to Susan Slusser of the SF Gate, as usual via mlbtraderumors.

Suffice it to say the 23 year old Gallagher has the most potential of the three pitchers in the Scott Hairston deal, though his stock has dropped due to injuries after being the centerpiece of the Rich Harden trade.

Sean tore up the minors, and put together a solid 104 tRA+ in more than 100 innings between Chicago and Oakland last year. This changes the face of the trade, and as I said before is exactly the kind of return we should want for Hairston. Gallagher could be an affordable 2nd or 3rd starter for years to come.

John Sickels has an additional scouting report. Sickels thinks Gallagher needs time to adjust to the majors, but will become an effective 3-4 starter.

PECOTA comps seem to back this up: Jeff Russell, Ryan Dempster, Jim Gott, Dave Borkowski, Moe Drabowsky, Brad Penny, Ray Culp, Tom Seaver (!), Geremi Gonzalez, Steve Dunning, Jose Rijo, and Mickey Lolich make up the top dozen, with Sidney Ponson, Dennis Martinez, and Jake Peavy also making appearances. Most of these guys were good pitchers, though their durability varied. A maximal, Seaver-like outcome seems unlikely, but there are some very good pitchers on this list, testifying to Gallagher’s potential. He’s not a PECOTA comp, but I could also see Jeff Suppan as an inning-eating model for Gallagher.