The process of becoming a gelding requires castration, and in this poem, Hard Rock is compared to "a freshly gelded stallion." Stallions are not typically gelded. A stallion is, by definition, a male horse that has not been gelded who is typically used instead to "stud" other horses; in the common vernacular, the term "stallion" has come to mean a powerful and masculine man. To compare a man to a gelded stallion, then, is to suggest that the man has been metaphorically deprived of his masculinity and, in this case, his source of power.
Castration in animals is associated with preventing aggression, and there is a suggestion in this poem that Hard Rock has been treated to make him less aggressive. In comparing the process of doctors who "bored a hole in his head" to that of castration, however, Hard Rock is described not only as if he is an animal—or being treated as such by the doctors—but also as if his bodily integrity is not his own. Like a horse who may be gelded by its owners, Hard Rock is subjected to treatments on his brain by the system that owns him. After his treatment, not only is he "tame," his mind has been utterly destroyed.