The mods gameplay varies depending on the map it is currently on. The two standard gameplay types are team based objective play which is accomplished by playing rounds and standard Deathmatch game play.
The objective team based gameplay consists of a particular team obtaining a number of goals/objectives in a series/order. Once the team has completed the all objectives then they will win the round and a new round starts again. Then depending on how the server is setup if the same team wins the next round (resulting in one team winning two out of three rounds) the server will inititate a map change which will start up another game of either objective or Deathmatch styles of game play.
The Deathmtach gameplay is a kill or be killed gameplay mode where everyone is against everyone regardless of their team designation. This game mode while being popular was not one that was originally intended for the mod because the official maps released with the mod are only team objective based maps. The Deathmatch gameplay also has a variable to it which is still a kill or be killed gameplay mode but it is one team verses the other team.
Most of the deathmatch style mapsrunning on the servers are ones that where ported from other Half Life mods or straight Deathmatch gameplay designed maps made by the SGTLS community map designers.
The Gate To Gates community created a special achievments server plug-in which when ran on a server incorperates additional SGTLS specific achivements that enhance the base game play.
In addition some of the communities mappers have developed maps which incorperate a combination of the two gameplay types or variances on them.
SGTLS gameplay no matter what game mode is always exciting and fun to play. If you haven't tried playing SGTLS yet we guarentee it's worth checking out, so install it and check it out today!