Their plan is to have the area surrounding the Gate be the arena for their final climactic battle with Team Evil. Getting there first gives them the home turf advantage, as they will have time to survey the area and prepare it for combat their way, without giving Xykon and friends the opportunity to do the same.

Right now, there is no long-term plan from the Order, because there simply is no long-term if they fail, the gods will just destroy the world. If they succeed in stopping Team Evil, then they can delay the destruction of the world for long enough to actually start planning a more permanent solution, but at this point it all hinges on protecting this Gate from any and all immediate threats at all costs.