Rockstar has made available a content creator (currently in beta version). To get the access to this application, you just need to download an automatic update to your console. The creator allows you to create custom games in several modes. You have, basically speaking, unrestrained freedom as to game location, available weapons (in Deathmatch), or setting whatever starting point for a race you can imagine.
Game modes available:
- Team Deathmatch
To start the creator, enter the game menu, go to Online tab and select Creator (BETA).