Jarun nudist beach and The island of Croatian Youth (Jarunska nudistička plaža i Otok hrvatske mladeži)
A gay beach does not exist in Zagreb. However, there is nudist part at the Jarun lake which is also known as a gay cruising area.
If you're looking for some more straightforward action, you should visit the island across the nudist beach - its forest, bushes and south beaches are known as Zagreb's prime place for open-sky fornicating, regardless of orientation :) Prime gay spot is the south-eastern forest. The island's name is Otok hrvatske mladeži (The island of Croatian youth). Unfortunately, the name has little to do with the average visitors' description :)
You can enter the island by crossing the bridge at its eastern end. If you're at the nudist beach, just follow the roller's/biker's road about 1 km towards east, and then at the crossroads keep on going straight ahead (ie don't turn right). The "gay forest" is on your left as soon as you cross the bridge.