Many would say that going to a gay bar or club would be the answer. This might be true to an extent because one will always meet a guy or two who has bravely entered the club to satisfy their curiosity.
This is an easy question. We will be at the places where so-called "straights" would go -- straight clubs, churches, colleges, sporting events, conservative/liberal political events, coffee houses, gyms and even in the workplaces. Usually one would have to see them more than just for a single event and become somewhat of acquaintances. Call it synergy if you wish, but greros have a way of eventually connecting with one another. The big giveaway is that the grero will generally be alone.