Efficacy and Safety of E/C/F/TAF (Genvoya®) in HIV-1/Hepatitis B Co-infected Adults

Key Inclusion Criteria: * Both Cohorts 1 and 2: * The ability to understand and sign a written informed consent form, which must be obtained prior to initiation of study procedures * HIV/HBV co-infected adult males and non-pregnant and non-lactating females * No evidence of hepatocellular carcinoma (HCC) or clinical or imaging evidence of cirrhosis (ascites, variceal bleeding, encephalopathy). \--- Subjects should have documentation of an abdominal ultrasound in the 12 months prior to screening, or an abdominal ultrasound at screening, demonstrating the absence of cirrhosis and HCC. * Acute Hepatitis A virus (HAV) immunoglobulin M (IgM) negative * Hepatitis C virus (HCV) Ab negative, or HCV Ab positive with negative HCV RNA * Hepatitis D virus (HDV) Ab negative, or HDV Ab positive with negative HDV RNA * Estimated glomerular filtration rate (eGFR) ≥ 50 mL/min according to the Cockcroft-Gault formula * CD4+ count of \> 200 cells/μL * Chronic HBV infection as defined by * HBsAg positive for ≥ 6 months Or * HBsAg positive at screening and either hepatitis B e antigen (HBeAg) or HBV DNA positive ≥ 6 months Or * At screening: positive total hepatitis B core antibody (HBcAb) and negative immunoglobulin M antibody to hepatitis B core antigen (HBcIgM) antibody, and * HBsAg positive, or * HBeAg positive, or * HBV DNA positive * Cohort 1 (HIV and HBV treatment naive) only: * No current or prior anti-HIV treatment, including antiretroviral medications received for prevention (PrEP), or post exposure prophylaxis (PEP) * No current or prior anti-HBV treatment * Plasma HIV-1 RNA level ≥ 500 copies/mL at screening * Screening HBV DNA ≥ 3 log10 IU/mL and \< 9 log10 IU/mL * Cohort 2 (HIV suppressed) only: * Receiving current antiretroviral regimen for at least 4 consecutive months * No current or prior regimen containing 3 active anti-HBV agents (i.e. cannot be on tenofovir alafenamide (TDF)/emtricitabine (FTC)/Entecavir or TDF/lamivudine(3TC)/Entecavir) * Maintained plasma HIV-1 RNA \< 50 copies/mL for 6 consecutive months prior to and at the time of the screening visit. Unconfirmed virologic evaluation of ≥ 50 copies/mL after previously reaching viral suppression (transient detectable viremia, or "blip") and prior to screening is acceptable * Documented positive HIV antibody test * Screening HBV DNA \< 9 log10 IU/mL Key Exclusion Criteria: * Females who are breastfeeding * Positive serum pregnancy test (female of childbearing potential) * Have an implanted defibrillator or pacemaker * Current alcohol or substance use * A history of malignancy within the past 5 years (prior to screening) or ongoing malignancy other than cutaneous Kaposi's sarcoma (KS), basal cell carcinoma, or resected, non-invasive carcinoma. * Received solid organ or bone marrow transplant * Any history of, or current evidence of, clinical hepatic decompensation (e.g., ascites, encephalopathy or variceal hemorrhage). * Significant bone disease (e.g., osteomalacia, chronic osteomyelitis, osteogenesis imperfecta, osteochondroses), or multiple bone fractures * Active, serious infections (other than HIV-1 infection) requiring parenteral antibiotic or antifungal therapy within 30 days prior to Day 1 * Subjects on hemodialysis, other forms of renal replacement therapy, or on treatment for underlying kidney diseases (including prednisolone, and dexamethasone) * Any other clinical condition or prior therapy that, in the opinion of the Investigator, would make the subject unsuitable for the study or unable to comply with the dosing requirements * Investigational agents (unless approved by Gilead Sciences). Participation in any other clinical trial without prior approval from the sponsor is prohibited while participating in this trial Note: Other protocol defined Inclusion/Exclusion criteria may apply.