Inclusion Criteria:  Type of Subject and Disease Characteristics  Subjects must have completed a prior GB002 PAH study and, in the opinion of the Investigator and Sponsor, have been compliant with study procedures and have completed treatment with IP through parent study end-of-treatment (EOT) visit.  Treatment with standard of care PAH disease-specific background therapies (stable dose).  Informed Consent  Review and signature of an IRB-approved informed consent form.  Exclusion Criteria:  Medical Conditions  Persistent and clinically significant systemic hypertension or hypotension.  Interval history of newly developed left-sided heart disease.  a. History of severe and untreated obstructive sleep apnea.  Potentially life-threatening cardiac arrhythmia with an ongoing risk. Uncontrolled bacterial, viral, or fungal infections which require systemic therapy. Other severe acute or chronic medical or laboratory abnormality that may increase the risk associated with study participation or GB002 administration or may interfere with the interpretation of study results and, in the judgment of the Investigator, would make the subject inappropriate for entry into this study. History of portopulmonary hypertension or portal hypertension classified as Child-Pugh Class A or higher. Subjects with a history of severe milk protein allergy. In addition, subjects with known intolerance or hypersensitivity to lactose who, in the opinion of the investigator, may experience severe symptoms following the ingestion of lactose. Current use of inhaled tobacco and/or inhaled marijuana. Ingestible or topical marijuana is allowed, per local restrictions and regulations. Documented history of alcohol abuse and/or history of utilizing drugs of abuse (amphetamines, methamphetamines, cocaine, phencyclidine [PCP]).  Have any other condition or reason that, in the opinion of the Investigator and/or the Sponsor's Medical Monitor (or designee), would prohibit the subject from participating in the study.  Diagnostic Assessments  Chronic renal insufficiency Hemoglobin (Hgb) concentration <8.5 g/dL. Absolute neutrophil count (ANC) < 1x 10^9/L.  Platelet count <50 x 10^9/L.  Prior Therapy  Use of inhaled prostanoids. Chronic use of oral anticoagulants (ie, coumadin or novel oral anticoagulant [NOAC]). Chronic use of any prohibited medication.  NOTE: Additional inclusion/exclusion criteria may apply, per protocol.