This open-label extension study will evaluate the long-term effects of GB002 (seralutinib) in subjects who previously participated in a GB002 PAH study.

Inclusion Criteria:

Type of Subject and Disease Characteristics

1. 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.

2. Treatment with standard of care PAH disease-specific background therapies (stable dose).

   Informed Consent

3. Review and signature of an IRB-approved informed consent form.

Exclusion Criteria:

Medical Conditions

1. Persistent and clinically significant systemic hypertension or hypotension.

2. Interval history of newly developed left-sided heart disease.

3. Potentially life-threatening cardiac arrhythmia with an ongoing risk.

4. Uncontrolled bacterial, viral, or fungal infections which require systemic therapy.

5. 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.

6. History of portopulmonary hypertension or portal hypertension due to cirrhosis classified as Child-Pugh Class A or higher.

7. 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.

8. Current use of inhaled tobacco and/or inhaled marijuana. Ingestible or topical marijuana is allowed, per local restrictions and regulations.

9. Current alcohol use disorder as defined by DSM-5, and/or history of current utilization of drugs of abuse (amphetamines, methamphetamines, cocaine, phencyclidine [PCP]).

10. 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

11. Chronic renal insufficiency

12. Hemoglobin (Hgb) concentration <8.5 g/dL.

13. Absolute neutrophil count (ANC) < 1x 10^9/L.

14. Platelet count <50 x 10^9/L.

    Prior Therapy

15. Use of inhaled prostanoids.

16. Chronic use of oral anticoagulants (ie, vitamin K antagonist such as warfarin or novel oral anticoagulant [NOAC]/direct oral anticoagulant [DOAC]).

17. Chronic use of any prohibited medication.

NOTE: Additional inclusion/exclusion criteria may apply, per protocol.