Study of GS-4321 in Healthy Participants and Participants With Chronic Hepatitis Delta Virus

Key Inclusion Criteria: Part A: * Participants assigned male or female at birth who are of childbearing potential and engage in heterosexual intercourse must agree to use protocol-specified method(s) of contraception. * Have a body mass index (BMI) of ≤ 30.0 kg/m2 at screening and at admission. Part B: * Participants assigned male or female at birth who are of childbearing potential and engage in heterosexual intercourse must agree to use protocol-specified method(s) of contraception. * Chronic hepatitis delta (CHD) for ≥ 6 months prior to screening, documented by prior medical history. * Must be receiving a commercially available entecavir, TAF, or TDF for the treatment of hepatitis B virus (HBV) infection at or prior to enrollment. Coformulation as part of a fixed-dose combination for the treatment of HIV is permitted. * Non-cirrhotic or compensated cirrhosis. * Hepatitis delta virus ribonucleic acid (HDV RNA ) \> 500 IU/mL at screening. * Alanine aminotransferase (ALT) level \> 1 × Upper limit of normal (ULN), but \< 10 × ULN at screening. Key Exclusion Criteria: Part A: * Positive serum or urine pregnancy test. * Participants with plans to breastfeed during the study period. Part B: * Positive serum or urine pregnancy test. * Participants with plans to breastfeed during the study period. * Current or previous clinically decompensated liver disease, including coagulopathy, hepatic encephalopathy, and esophageal varices hemorrhage due to HDV or HBV. * Child-Turcotte-Pugh (CTP)-B or -C or a CTP score of ≥ 7. Note: Other protocol defined Inclusion/Exclusion criteria may apply.