Firstly, you're going to need to define the area which will be your arena. For the arena, it needs to be an enclosed cuboid, and have no roof (it may have a glass roof, but no other material). It should also have an area for spectators. Once you have your arena, select it with World Edit in the usual manner and use the command /gr setarena - You should get a confirmation message.
Now you're going to need to set the spawn points for the lobby, red team, blue team, and spectator area. To do this, simply stand where you want the spawn point to be for each, and use the command /gr setspawn [lobby|red|blue|spectator] (For example, to set the lobby spawn, stand where you want the lobby spawn to be, and type "/gr setspawn lobby").
Next you need to set up the signs that GoldRush will use to assign players to teams. You need to have 2 signs in your lobby somewhere, they should be blank (if they have text on them, the text will be overwritten). Simply use the command /gr setsign [red|blue] and right-click the sign for that team. (You need to do this twice, once for the red sign using "/gr setsign red" and once for the blue sign using "/gr setsign blue". Optionally, you can also add class signs that players can Right-Click to select their class. To do this, simply write "[GoldRush]" on the first line of the sign, and the class name (not case sensitive) on the second line. Simple!
Technically, you're good to go now - however, if you want a truly unique GoldRush experience, then I recommend you customise your configuration files. To find out how, click this link: click me!