I would say that it is third person limited (the narrator only knowing the thoughts and feelings of a single character, in this case, TS Garp.)

However, Irving complicates matters by toggling between two third person limited points of view:  that of Garp, ("Garp found that he could forget her; lust, as his mother called it, was tricky that way")  and that of his "official biographer"  ("In the world according to Garp, we are all terminal cases.")