Sitges gay beach is right in the center of town. Playa De la Bassa Rodona.
There is a very popular nudist beach with a gay section 15 minutes walk from town. Playa de las Balmins.
An exclusively gay nudist beach and busy cruising zone are a 50 minute walk from town. If you take a taxi, you still have to walk for 20 minutes. Playa del Muerto.

It’s worth noting, every hotel in Sitges is Gay Friendly, if you can’t get into any of the above.
Sitges has been a popular gay retreat for over 100 years, making it a part of the town’s culture.