Gay and bisexual men (GBM) are a key population affected by human immunodeficiency virus (HIV) and hepatitis C virus (HCV) coinfection. We aimed to measure HCV treatment effectiveness and to determine the population impact of treatment scale-up on HCV prevalence and incidence longitudinally among GBM. The co-EC Study (Enhancing Care and Treatment Among HCV/HIV Coinfected Individuals to Eliminate Hepatitis C Transmission) was an implementation trial providing HCV direct-acting antiviral treatment in Melbourne, Australia, during 2016-2018. Individuals with HCV/HIV coinfection were prospectively enrolled from primary and tertiary care services. HCV viremic prevalence and HCV antibody/viremic incidence were measured using a statewide, linked, surveillance system. Among 200 participants recruited, 186 initiated treatment during the study period. Sustained virological response in primary care (98% 95% confidence interval {CI}, 93%-100%) was not different to tertiary care (98% 95% CI, 86%-100%). From 2012 to 2019, between 2434 and 3476 GBM with HIV infection attended our primary care sites annually, providing 13 801 person-years of follow-up; 50%-60% received an HCV test annually, and 10%-14% were anti-HCV positive. Among those anti-HCV positive, viremic prevalence declined 83% during the study (54% in 2016 to 9% in 2019). HCV incidence decreased 25% annually from 1.7/100 person-years in 2012 to 0.5/100 person-years in 2019 (incidence rate ratio, 0.75 95% CI, .68-.83; P < .001). High treatment effectiveness by nonspecialists demonstrates the feasibility of treatment scale-up in this population. Substantial declines in HCV incidence and prevalence among GBM provides proof-of-concept for HCV microelimination. NCT02786758.