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Cosloy was raised in Wayland, Massachusetts, a western suburb of Boston. While he was in high school, he became involved in the local hardcore punk scene, put together many punk shows, and started Conflict, a mimeographed fanzine of underground and alternative rock and art. He continued to write and edit the fanzine during the '80s and early '90s.
He ran Homestead Records in the 1980s; DJ'd at WFMU; played in the group Air Traffic Controllers; and later became part owner of Matador Records, the New York independent label founded by Chris Lombardi.
In 1984 he was approached by the management of Dutch East India Trading Co., a Long Island-based record distributor, to manage an independent record label previously started by Sam Berger called Homestead. He moved to New York for the position. Homestead put out many of the mid-80s defining alt-rock records, from Big Black, Sonic Youth, Dinosaur Jr., My Dad Is Dead, The Membranes and many others. He continued to publish Conflict during this time.
It was via Conflict that Cosloy first heard the band Pavement, after Scott Kannberg sent the fanzine the band's first studio recording, Slay Tracks. He did some DJing at WZBC, Boston College's radio station, as well. He had a record label called Conflict Records, which issued two compilations called Bands that Could be God I and II. He also put out a few singles. Gerard also promoted shows in the Boston area, one of them being hardcore band Deep Wound which evolved into Dinosaur Jr.
Matador Records became Cosloy's next label to work with. Matador Records was started in 1989 by Dutch East employee Chris Lombardi and Cosloy joined into the partnership in 1990.
For a time Cosloy lived in London working with Matador's UK and US operations. As of Autumn 2004, he has been living in Austin, Texas, continuing his partnership with Lombardi.
Cosloy is also a musician and is a member of Air Traffic Controllers as well as working with punk singer GG Allin for a short period. From 1992-1996, he fronted the New York quartet Envelope, while briefly filling in as bassist for the Dustdevils during Mark Ibold's tenure in Pavement.
Cosloy has a daily mostly-sports blog called Can't Stop the Bleeding. 
Cosloy's house was destroyed by fire on August 11, 2009. Cosloy, who worked out of his home, and another resident were awakened and evacuated around 3 a.m. by an officer of the Austin Police Department. Cosloy was quoted as saying, "There are a lot of people who have a lot less than I do who deal with a lot worse, but this is pretty bad."