A Phase 3, Open-label Study to Investigate the Efficacy and Safety of Sofosbuvir Plus Ribavirin in Chronic Genotype 1, 2, 3 and 4 Hepatitis C Virus (HCV) and Human Immunodeficiency Virus (HIV) Co-infected Adults

Inclusion Criteria: * Age ≥ 18 years with HIV-1 and chronic HCV genotype 1, 2, 3, or 4 co-infection * HCV RNA \> 10,000 IU/mL at screening * HCV treatment history: * Treatment-naive for HCV genotypes 1, 2, 3, or 4, or * Treatment-experienced for HCV genotypes 2 or 3 * HIV antiretroviral (ARV) criteria: * On a stable, protocol-approved, HIV ARV regimen with undetectable HIV RNA for \> 8 weeks prior to screening, or * ARV untreated for ≥ 8 weeks prior to screening, with a CD4 T-cell count \> 500 cells/mm\^3 * Presence or absence of cirrhosis; a liver biopsy may be required * Healthy according to medical history and physical examination with the exception of HCV and HIV diagnosis * Agree to use two forms of highly effective contraception for the duration of the study and 6 months after the last dose of study medication Exclusion Criteria: * HCV genotype 1 or 4 with previous HCV treatment * Poor control with HIV ARV regimen requiring a possible dose modification of therapy within 4 weeks of study medication dosing * A new AIDS-defining condition diagnosed within 30 days prior to screening * Prior use of any other inhibitor of the HCV NS5B polymerase * History of any other clinically significant chronic liver disease * Evidence of or history of decompensated liver disease * Chronic hepatitis B virus (HBV) infection * Hepatocellular carcinoma (HCC) or other malignancy (with exception of certain resolved skin cancers) * Chronic use of immunosuppressive agents or immunomodulatory agents * Clinically relevant drug or alcohol abuse within 12 months of screening * History or current evidence of any condition, therapy, laboratory abnormality or other circumstance that might confound the results of the study, or interfere with the participant's participation for the full duration of the study or not be in the best interest of the participant in the opinion of the investigator