In the 1860s, San Franciscans began to feel the need for a spacious public park similar to Central Park that was taking shape in New York. (Wikipedia) For the source and more detailed information...
No. The Presidio is some miles North of Golden Gate park. The park is actually in the West of the City and the Presidio is in the North of the City.
San Francisco's largest park, Golden Gate Park covers over 1,013 acres and is 3 miles long by 1/2-mile wide.
Golden Gate Park.
There are several parks, towns and cities named "Union Park," so this question cannot be answered until you specify which one you are talking about.