Act 2 Scene 1, What rumor of political upheaval reaches the castle of Gloucester?
I believe that what you are talking about is the news about a possible war. This news comes to Edmund, telling him that there is a good possibility that the Dukes of Albany and Cornwall will soon be at war.
This news is not all that important to the plot. But the Duke of Cornwall is also going to be coming to Gloucester's castle. That is what is really relevant here. That is because Edmund is going to use the coming of Cornwall to help him out with his plot against his brother Edgar.