Inclusion Criteria:

• A current diagnosis of symptomatic PAH classified by one of the following:
  • Idiopathic PAH (IPAH) or heritable pulmonary arterial hypertension (HPAH);
  • PAH associated with connective tissue diseases;
  • PAH associated with anorexigen or methamphetamine use;
  • Congenital heart disease with simple systemic to pulmonary shunt at least 1 year after surgical repair.
• 6MWD ≥ 150 meters and ≤ 550 meters at screening.
• WHO FC II or III symptomatology.
• Treatment with standard of care PAH background therapies.
• Documentation of cardiac catheterization within the screening period that is consistent with the diagnosis of PAH and meeting all the following criteria, to be confirmed by a central hemodynamic core laboratory:
  • Mean pulmonary arterial pressure (mPAP) ≥ 25 mmHg (at rest);
  • Pulmonary capillary wedge pressure ≤ 15 mmHg, or mean left atrial pressure (mLAP) or left ventricular-end diastolic pressure (LVEDP) ≤ 15 mmHg in the absence of left atrial obstruction;
  • PVR ≥ 400 dyne•s/cm^5.
• Pulmonary function tests (PFTs) and diffusing capacity of the lungs for carbon monoxide (DLCO) at screening with the following criteria met:
  • Forced expiratory volume in 1 second (FEV1) ≥ 60% (predicted);
  • DLCO ≥ 40% predicted except for subject with PAH associated with systemic sclerosis (SSc-APAH) where DLCO ≥ 30% is required.

Exclusion Criteria:

• Evidence of chronic thromboembolic disease or acute pulmonary embolism as assessed by ventilation-perfusion (V/Q) scan, computed tomography (CT)-angiogram, or pulmonary angiogram prior to screening.
• Uncontrolled systemic hypertension as evidenced by sitting systolic blood pressure > 160 mm Hg or sitting diastolic blood pressure > 100 mm Hg during screening visit after a period of rest.
• Systolic blood pressure < 90 mm Hg during screening and baseline visits.
• WHO Pulmonary Hypertension Group 2-5.
• Human immunodeficiency virus (HIV)-associated PAH.
• History of left-sided heart disease and/or clinically significant cardiac disease.
• Untreated obstructive sleep apnea.
• History of atrial septostomy within 180 days prior to screening.
• Pulmonary venous occlusive disease (PVOD).
• History of portopulmonary hypertension or portal hypertension classified as Child-Pugh Class A or higher; or baseline ALT or AST > 2 x ULN or Total Bilirubin ≥ 2 X ULN at screening.
• History of malignancy within 5 years prior to screening with the exception of localized nonmetastatic basal cell carcinoma of the skin and in-situ carcinoma of the cervix.
• History of a potentially life-threatening cardiac arrhythmia with an ongoing risk.
• Severe acute or chronic medical or laboratory abnormality that may increase the risk associated with study participation or IP administration (eg; history intracranial hemorrhage).
• Chronic renal insufficiency as defined by an estimated glomerular filtration rate (eGFR) ≤ 30 mL/min via Chronic Kidney Disease Epidemiology Collaboration (CKD-epi) at screening or requires dialytic therapy or hemofiltration.
• Hemoglobin (Hgb) concentration < 8.5 g/dL at screening.
• Evidence of active HIV, Hepatitis B or Hepatitis C, or tuberculosis (TB) infections.
• Inhaled prostanoids; these drugs must be withdrawn prior to or at screening.
• Use of anticoagulants (ie, coumadin or NOAC) at randomization.
• Requirement of intravenous (IV) inotropes (i.e., levosimendan, dopamine, dobutamine, milrinone, norepinephrine) other than an IV prostanoid within 4 weeks of screening.
• Prior participation in GB002 studies and/or prior treatment with GB002.
• Currently participating in or has participated in a study of an investigational agent or has used an investigational device for the treatment of PAH within 4 weeks prior to screening.
• Current use of inhaled tobacco and/or marijuana.
• History of severe milk protein allergy or known intolerance or hypersensitivity to lactose and experience severe symptoms following ingestion.
• A positive test for drugs of abuse (amphetamines, methamphetamines, cocaine, phencyclidine [PCP]).
  • NOTE: Additional inclusion/exclusion criteria may apply, per protocol.