The social cognitive deficits associated with autism spectrum disorder (ASD) are related to an imbalance in excitatory and inhibitory neurotransmission, specifically a deficit in the inhibitory neurotransmitter GABA. The investigators have used magnetic resonance spectroscopy (MRS) techniques to measure GABA in specific brain regions and have demonstrated that a single dose of gabapentin increases GABA in brain regions associated with social cognition. This study will use a biomarker-driven approach to investigate gabapentin to correct the underlying imbalance of neurotransmitters and improve the core social cognitive deficits in ASD. By using a brain-based biomarker (GABA) that is quantifiable and measurable, the investigators can target this biomarker directly and measure the impact of the treatment. This will help with the future development of targeted therapies for ASD and provide an early marker of response to aid in the selection of individuals more likely to respond to various treatments. The specific aims of this study are to: 1) determine if treatment with gabapentin sustainably increases GABA in the right anterior insula (RAI; an area of the brain involved in social cognition), 2) determine if response of RAI GABA levels to a single dose challenge of gabapentin predicts a sustained response after treatment, and 3) determine if the increase in GABA levels with gabapentin treatment translates into clinically measurable improvement in social cognition. The investigators will conduct an 8-week open-label clinical trial of gabapentin in 40 adolescents (age 13-17 years) with ASD, using MRS before and after treatment to measure GABA in the RAI (the primary outcome for the study). Before the trial, a single dose challenge of gabapentin will be used to evaluate the immediate response of GABA levels in the RAI, to determine if this predicts later response. A secondary outcome will be the clinical effects of gabapentin on social cognition. This study can demonstrate for the first time that neuroimaging biomarkers can be used to guide treatment of social cognition deficits seen in ASD and that the excitatory-inhibitory imbalance in neurotransmitters in ASD can be pharmacologically targeted. This can provide a rational basis for pharmacological treatment of the core social deficits of ASD, providing direct benefit to participants in the study as well as indirect benefit to countless patients in the future.