Gongaga is a small village surrounded by trees. Most of the Village was destroyed when the Mako reactor blew up. The village itself has a few shops and a place to rest.
When you enter Gongaga you will encounter Reno and Rude (see below). Once these guys have disappeared head right at the first junction, this should take you towards the destroyed reactor. When you get there some more Turks will turn up, once they have gone, examine the area that they were looking at, you should find the Red Titan Summon Materia.
Now head back the way you came, and follow the path into the main Village. Once you have finished in the village, leave Gongaga and head south and round towards Cosmo Canyon.
When you enter Gongaga you meet Reno and Rude.