Inclusion Criteria:  Ages 18-65 years Meet DSM-5 criteria for moderate or severe cannabis use disorder (CUD; within the past 3 months), provide a positive urine cannabinoid screen at baseline, and identify cannabis as the primary substance of abuse Meet DSM-5 criteria for bipolar I or II disorder (BD) or Schizoaffective Disorder, Bipolar Type Able to provide informed consent and read, understand, and accurately complete assessment instruments Willing to commit to medication treatment and follow-up assessments Prescribed daily use of at least one mood stabilizing medication (i.e., lithium, divalproex sodium, lamotrigine, carbamazepine, 2nd generation antipsychotic)  Exclusion Criteria:  A primary psychiatric diagnosis other than BD (e.g., Schizophrenia) Meet DSM-5 criteria for moderate or severe substance use disorder (other than cannabis or tobacco) within the past 60 days Any uncontrolled neurological condition (e.g., epilepsy) that could confound the results of the study Any history of brain injury with loss of consciousness greater than 5 minutes Any history of mental retardation, dementia, or recent electroconvulsive therapy (in the past 3 months) Any uncontrolled medical condition that may adversely affect the conduct of the study or jeopardize the safety of the participant Hepatocellular disease as indicated by plasma levels of liver transaminases (aspartate transaminase, alanine transaminase) greater than 3 times the normal range Renal insufficiency as indicated by plasma levels of creatinine greater than 2 times the normal range Concomitant use of medications that could interfere with glutamatergic/GABAergic transmission (e.g., benzodiazepines, ceftriaxone, riluzole, memantine, ketamine, topiramate, vigabatrin), due to potential confounding effects Concomitant use of opioid medications, benzodiazepines, barbiturates, chloral hydrate, sodium oxybate, or any other medication deemed to be hazardous if taken with gabapentin Azelastine, orphenadrine, oxomemazine, paraldehyde, and thalidomide are generally contraindicated in patients taking gabapentin; as such, individuals taking these medications will be excluded Women of childbearing potential who are pregnant, lactating, or refuse adequate forms of contraception Current suicidal or homicidal risk Baseline scores greater than 35 on the Montgomery-Asberg Depression Rating Scale or greater than 25 on the Young Mania Rating Scale Has taken gabapentin in the last month or experienced adverse effects/allergic reaction (e.g., angioedema) from it at any time Significant claustrophobia and/or past negative experiences with MRI Presence of non-MRI safe materials in the body (e.g., ferrous metal implants, pacemaker)