Tim Gill plans to use the proceeds from the sale to benefit the Gill Foundation, an organization he established in 1994 to benefit gay, lesbian, bisexual and transgender causes. At the end of last year, the foundation had an endowment of $163 million.
"I am proud of the work of Quark and expect it to maintain its position as a premier developer of innovative software," Gill said in a statement. "But my passion now lies in philanthropy."
Denver-based Quark, which is best known for selling Quark XPress software used for page layout, has not yet named a new chairman. However, Quark spokesman Glen Turpin said that vice president Kamar Aulakh will largely fill Gill's role as the head of the engineering staff.
Gill's departure, announced to employees on Monday, was not unexpected.
"Tim has been stepping back from daily operations for quite some time," Turpin said.
Turpin would not say whom Gill sold his stake to. However, The Denver Post reported that it was sold to Quark CEO Fred Ebrahimi, who bought a 50 percent stake in privately held Quark from Gill in 1986.
Gill started Quark in 1981 with a $2,000 loan from his parents. The company currently has 700 employees.
The Gill Foundation was launched two years after Colorado voters passed Amendment 2, a law that would have prevented local governments from adopting laws protecting gay rights. The U.S. Supreme Court struck down the law in 1996.