Gerber and Buck generally produce their knives in 425M stainless, or occasionally in ATS-34 for their higher end products. They do this so they can keep costs relatively low, and compete with companies based in Taiwan such as CRKT.
As for the fixed blades produced by Gerber, I would assume that it is in 425M, because the only ATS-34 knives from Gerber, to my knowledge, are the ATS-34 version of the EZ-OUT series, and the Covert.
Whats wrong with 425M? Nothing is really wrong with it, but it is relatively soft compared to the other steels, and is rather below average in edge holding ability.
Gerber's fixed blades are nice, but expect to be sharpening more often if you use your knife for anything more than boxes, the mail, and light cutting.