At least it didn't cost a prospect this time, as the Pirates acquired Gallagher for cash. I liked Gallagher coming through the minors because he posted good numbers and was pretty young for his levels - he pitched very well at Class AA as a 20-year-old, which isn't easy to do. He hasn't progressed much since then, and has consistently struggled in the majors. He's only 24, and he can start, so he does have value, but he's a little bit like Dana Eveland in that the most interesting things about him have to do with the way he played many years ago. He was a key component in the Rich Harden trade a couple of years back, which means that the A's thought pretty highly of him then. He throws a fastball that's generally around 90-93 MPH, plus a curve, slider and change.
Here's a pretty good article on Gallagher's stuff. The author thinks that if Gallagher might have more success if he stops throwing his slider and concentrates on his curveball as his breaking pitch. It does appear that he's changed a lot since he got his start in the big leagues - he came up as a fastball/curve guy, but then began throwing more and more sliders and changeups, with no success. He's young enough that he might have some upside if he changes his approach.
The Pirates will have to remove a player from the 25-man roster to make way for Gallagher - Justin Thomas might be the one who goes.