The primary objectives of Cohort 1 are to evaluate the steady state pharmacokinetics (PK) for elvitegravir (EVG) and tenofovir alafenamide (TAF) and confirm the dose of the elvitegravir/cobicistat/emtricitabine/tenofovir alafenamide (E/C/F/TAF) STR (Part A) and to evaluate the safety and tolerability of E/C/F/TAF STR through Week 24 (Part B) in human immunodeficiency virus - 1 (HIV-1) infected, antiretroviral (ARV) treatment-naive adolescents. The primary objectives of Cohort 2 are to evaluate the PK of EVG and TAF (Part A), and to evaluate the safety and tolerability of E/C/F/TAF through Week 24 (Part B) in virologically suppressed HIV-1 infection children 6 to \< 12 years weighing \>= 25 kg. The primary objectives of Cohort 3 are to evaluate the PK of EVG and TAF and confirm the dose of the STR, and to evaluate the safety and tolerability of E/C/F/TAF low dose (LD) STR in virologically suppressed HIV-1 infected children ≥ 2 years of age and weighing ≥ 14 to \< 25 kg.
Key Inclusion Criteria: * Cohort 1 * Age at baseline: 12 years to \< 18 years old * Weight at screening: ≥ 35 kg (77 lbs) * Plasma HIV-1 ribonucleic acid (RNA) levels of ≥ 1,000 copies/mL at screening (Roche COBAS TaqMan v2.0) * Screening genotype report shows sensitivity to EVG, emtricitabine (FTC) and tenofovir (TFV) * No prior use of any approved or experimental anti-HIV-1 drug for any length of time * Cohort 2 * Age at baseline: 6 years to \< 12 years old * Weight at screening: ≥ 25 kg (55 lbs) * Plasma HIV-1 RNA of \< 50 copies/mL (or undetectable HIV-1 RNA level according to the local assay being used if the limit of detection is \> 50 copies/mL) for ≥ 180 consecutive days (6 months) prior to screening on a stable antiretroviral regimen, without documented history of resistance to any component of E/C/F/TAF STR. * Cohort 3 * Age at baseline: ≥ 2 years old * Weight at screening: ≥ 14 kg (31 lbs) to \< 25 kg (55 lbs) * Plasma HIV-1 RNA: \< 50 copies/mL (or undetectable HIV-1 RNA level according to the local assay being used if the limit of detection is \> 50 copies/mL) for ≥ 180 consecutive days (6 months) prior to screening on a stable antiretroviral regimen, without prior history of resistance to any component of E/C/F/TAF STR Key Exclusion Criteria: * Hepatitis B or hepatitis C virus infection * Evidence of active pulmonary or extra-pulmonary tuberculosis disease within 3 months of the screening visit. * Individuals experiencing decompensated cirrhosis * Pregnant or lactating females Note: Other protocol defined Inclusion/Exclusion criteria may apply.