Gary is a serial killer who targeted young women in the Philadelphia area.
He keeps the heads of his victims in the freezer at his apartment. His targets are said to be young, attractive blondes. One could interpret this as a representation of his frustration with Dee for failing to become romantically involved with her, her inability to water his plants, or a mix of the two. Furthermore, the fact that Dee has never watered his plants, potentially avoiding a trap in waiting by Gary, could mean that she has escaped becoming another victim simply by way of laziness.