Gaithersburg is a city in Montgomery County, Marylan. As of 2005, the city had an estimated total population of 57,698, making it the fourth largest in the state behind Baltimore, Rockville, and Frederick. This city is located at 39°8' North, 77°13' West, to the northwest of Rockville, the county seat of Montgomery County. Gaithersburg was incorporated in 1878.