Study to Evaluate Switching From a TDF-Containing Combination Regimen to a TAF-Containing Fixed Dose Combination (FDC) in Virologically-Suppressed, HIV-1 Positive Participants

Key Inclusion Criteria:

* Ability to understand and sign a written informed consent form, which must be obtained prior to initiation of study procedures
* Currently receiving antiretroviral therapy consisting of E/C/F/TDF, EFV/FTC/TDF, RTV+ATV+FTC/TDF, or COBI+ATV+FTC/TDF for ≥ 6 consecutive months preceding the final visit in their earlier study
* Completion of the Week 144 visit in studies GS-US-236-0102, GS-US-236-0103, GS-US-216-0114, or completion of the Week 96 visit in study GS-US-264-0110 (only participants on an EFV-based regimen), or completion of studies GS-US-236-0104, GS-US-216-0105
* Plasma human immunodeficiency virus type 1-ribonucleic acid (HIV-1 RNA) concentrations at undetectable levels for at least 6 consecutive months prior to the screening visit and have HIV RNA \< 50 copies/mL at the screening visit
* Normal echocardiograph (ECG)
* Estimated glomerular filtration rate (GFR) ≥ 50 mL/min according to the Cockcroft-Gault formula for creatinine clearance
* Hepatic transaminases (aspartate aminotransferase \[AST\] and alanine aminotransferase \[ALT\]) ≤ 5 × upper limit of the normal range (ULN)
* Direct bilirubin ≤ 1.5 x ULN
* Adequate hematologic function
* Serum amylase ≤ 5 × ULN
* Females of childbearing potential must agree to utilize highly effective contraception methods or be non-heterosexually active or practice sexual abstinence from screening throughout the duration of study treatment and for 12 weeks following the last dose of study drug if receiving EFV/FTC/TDF regimen, and 30 days for those assigned to all other regimens.
* Female participants who utilize hormonal contraceptive as one of their birth control methods must have used the same method for at least three months prior to study dosing
* Female participants who have stopped menstruating for ≥ 12 months but do not have documentation of ovarian hormonal failure must have a serum follicle stimulating hormone (FSH) level at screening within the post-menopausal range based on the Central Laboratory reference range

Key Exclusion Criteria:

* A new acquired immunodeficiency syndrome (AIDS)-defining condition diagnosed within the 30 days prior to screening
* Hepatitis B surface antigen position
* Hepatitis C antibody positive
* Participants experiencing decompensated cirrhosis
* Females who are breastfeeding
* Positive serum pregnancy test
* Have an implanted defibrillator or pacemaker
* Current alcohol or substance use judged by the Investigator to potentially interfere with subject study compliance
* History of malignancy within the past 5 years or ongoing malignancy other than cutaneous Kaposi's sarcoma, basal cell carcinoma, or resected, noninvasive cutaneous squamous carcinoma
* Active, serious infections (other than HIV-1 infection) requiring parenteral antibiotic or antifungal therapy within 30 days prior to baseline
* 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 dosing requirements
* Participation in any other clinical trial without prior approval from the sponsor is prohibited while participating in this trial
* Participants receiving ongoing therapy with drugs not to be used with elvitegravir (EVG), COBI, FTC, TDF, ATV, RTV, EFV, and TAF or participants with any known allergies to the excipients of E/C/F/TDF, E/C/F/TAF, EFV/FTC/TDF, ATV, COBI, RTV, or FTC/TDF

Note: Other protocol defined Inclusion/Exclusion criteria may apply.