GP4s are better tires but much narrower than spec'd. The 28s measure only 26 mm wide on my rims, Mavic Open Pros.

Personally, I think a better choice would be the Vittoria Voyager Hyper 32s, which have folding beads and a reflective sidewall, and roll very nicely. For less money, Panaracer Pasela PTs are another fine choice. IMHO, Contis are overpriced and always run narrow, which is not what I want in a commuter tire.