Gaastra is a city in Iron County in the U.S. state of Michigan. As of the 2010 census, the city population was 347, making it Michigan's third-…Gaastra is a city in Iron County in the U.S. state of Michigan. As of the 2010 census, the city population was 347, making it Michigan's third-smallest incorporated city by population after Omer and Lake Angelus. In 2020, its population was 316.