There may not be a thoroughly gay establishment but there is a stretch of beach where locals and farang cruise. It is named Outres Beach, about 1.5 km south (?) of The Snook. In fact, there is a club on Planet Romeo with a small but devoted membership called Sihanoukcruisin. Herewith its blurb:

Club for guys from Sihanoukville who like to have sex on the beach. On Otres-Beach are 1.5 km free beach space in the nature.
Join this club to exchange tips on where to have sex, and to find guys who are open to meet at such places :)

So what is Cruising?
Cruising is the act of walking or driving about a locality in search of a sex partner, usually of the anonymous, casual, one-time variety.

Men looking for other men on the Otres-Beach in Sihanoukville and having sex together. So some spot near the beach are very public, so take care, to do it discret.