Stargate SG-1 is a television series originally on Showtime and then on the Sci-Fi Channel. It follows the adventures of a group of military people and scientists who possess an alien stargate which allows them to travel to other worlds.
Sheldon uses a Stargate episode as an example of a "tactical retreat", which was the strategy he suggests they use while playing paintball. He describes the episode as being "where they found themselves on a planet with a culture based loosely on Earth's Athens and Sparta". The others stare at him blankly and he realises they didn't see that episode.
Which is hard to believe, quite honestly.