where is union gap?
Union Gap, Washington. Union Gap redirects here. For the music group, see Gary Puckett & The Union Gap. Union Gap is a city in Yakima County, Washington, United States. As of the 2010 census, the city population was 6,047. Union Gap has a storied history and in recent years has become the retail hub for the entire Yakima Valley as a result of Valley Mall and other thriving businesses being located here.