Phelan-McDermid Syndrome (PMS) is a rare genetic condition characterized by deletion or mutation of region 22q13.3, which includes the SHANK3 gene. This population is clinically defined as having mildly dysmorphic features, epilepsy, neonatal hypotonia, severely impaired or absent expressive language, developmental delays, and intellectual impairments. Among these characteristics, individuals with PMS notably have autistic-like traits, which include abnormal reactivity to sensory stimuli. However, there are few if any EEG studies done on this population with regard to sensory processing. Therefore, this study focuses on event-related potential (ERP) comparisons between PMS and controls in a standard auditory gating task that measures attenuation of neural activity to repetitive auditory responses. There were 52 participants (27 females), 37 of which have PMS. Specific comparisons of interest include age, gender, and genotype between and within the PMS and control groups. Data analysis included a series of linear models using a region-specific and a global (whole-head) approach to characterize neural activity. The most notable findings between PMS and controls were in genotype, where PMS showed worse gating than controls for the P50 ERP. Within PMS, larger deletion sizes were associated with increased auditory processing abnormalities, especially in younger individuals, suggesting the possibility for developmentally regulated involvement of additional genes in this region. Results suggest that PMS exhibit auditory processing abnormalities that show complex variation by deletion-size, gender, and age, which may provide valuable insight into clinical characterization of sensory and speech behaviors in future studies.