No. The Giant Garter Snake is a recently recognized species, Thamnophis gigas, formerly considered a subspecies of what was then called the Western Aquatic Garter Snake, T. couchii. Nowadays, T. couchii is called the Sierra Garter Snake and the Western Aquatic Garter Snake is T. atratus. It's a little confusing. The San Francisco Garter Snake is a form of the Common Garter Snake, T. sirtalis.
The Giant and San Francisco Garter Snakes are completely allopatric, meaning their ranges do not overlap. This was probably the case even before their ranges were reduced. The Giant Garter Snake now occurs only in the central Central Valley and is considered Threatened; the San Francisco Garter Snake now occurs only on the San Francisco Peninsula and is considered Endangered.