I'm with foobar75, Bogut is a top 5 among defensive centers, with Dwight Howard in the top spot. Th problem with Bogut is he doesn't have much confidence in his offensive game, not to mention being so mediocre at the stripe (44%). If Bogut possessed a mid-range game (10-15 FT), better at the stripe (70-75%), and had a pass-first PG (Rondo, Kidd, Paul, Nash), his offensive numbers would be on par with that of Pau Gasol.
Marc Gasol is a solid defensive center as well, I'd consider him in the top 10 defensive centers. Marc is better than Bogut at the stripe (75%), but he doesn't get much shots, and if he had pass-first PG's as previously mentioned, he'd be a borderline All-Star level player.
Gortat is by far the best of the 3, who just needs more experience and playing time. Unlike, Bogut and M. Gasol, Gortat is benefiting from a true passing PG (Nash), and a raw player in Robin Lopez. He keeps getting better in each passing game. The Polish Hammer's got a good touch at the stripe (76%), a decent mid-range game (10-15 FT), and could block shots among the best.