If in San Francisco, the beach right under the Golden Gate Bridge is nude and -- well, it used to be all gay before the Park Service "improved" the pathways down.
South of San Francisco, there is a private gay nude beach just north of San Gregorio State Park. It's an unmarked entrance -- you can find directions online.
For beaches north of Santa Cruz, see https://www.santacruz.com/guides/santa-cruz-nude-beaches. Bonny Doon is quite nice, albeit mixed gay/straight. Laguna Creek is mostly gay, and quite pretty.
North of San Francisco, the most wonderful gay beach in California (take that, San Diego) used to be Black Sands Beach. THere's a parking lot down to the mixed straight/gay part, but further up the road, there used to be a trail down to the gorgeous gay beach. Last time I was there (two years ago), the trail had eroded to the point that it was quite dangerous to descend.