Kevin Garnett's primary position was power forward. Their previous centers if I can recall were Shaq and Jermaine O' Neal.
Perkins, too.
Kevin Garnett is indeed their starting center and lead rebounder (at only 7.4 rebounds/game though) right now. But yes, his primary position is power forward. There's been a dearth of "true" centers in the NBA in the past decade or so with an increasing number of big men playing more explosive games (e.g. Blake Griffin, Amar'e Stoudemire), so it's not surprising Garnett's been switched over to the center position given his height, length, and defensive prowess. Besides, the positions can be interchangeable (e.g. Tim Duncan).