The acting police chief of Gaithersburg was arrested yesterday by Montgomery County police and charged with stealing seven guns that had been seized by the Gaitersburg City Police Dept.

David Douglas Kincaid, a 32-year-old sergeant, was arrested at the county police station in Gaithersburg and released under pesonal bond charged with theft in excess of $300 , a felony. He was placed on administrative leave with pay.

Kincaid, who lives in Rockville and has been on the city police force since June 1978, was named the police department's acting head by former chief John Devries, who retired Friday for health reasons.

County police began their investigation at the request of the city police department on Aug. 28 when seven handguns, rifles and shotguns in the custody of the Gaithersburg police were discovered missing.

After an investigation by the county police department's Internal Affairs Division, property detectives charged Kincaid with selling some of the guns to a local gun dealer. The acting chief still had some of the guns in his possession, county police spokesman Nancy Moses said.

Kincaid had formerly worked as a state policeman in Rockville barracks, and for the Rockville City Police. "His record was satisfactory while he was here," one state policeman said.

A preliminary hearing is set for Dec. 12.