From the guide:
Guard Stations
Unlike Forts, Guard Stations will not make you less vulnerable to attacks. Nevertheless, they will effectively protect all the resources you own – land included –, making the enemy’s attacks against your province less efficient.
Decrease Resource Losses when Attacked by % * 2
From this blurb and from the attacks I have made and suffered, GS help on all attack types, learn, TM, mass, and plunder. Not sure about conquest and but definitely not ambush.
I am in a warring kingdom and I am usually one of the most hit provinces in my kingdom during war, I run 26% GS for this reason. Before war when we get mass waved or learn waved I lose very little resources.
In a kingdom that doesnt war much a good combo of TG and Forts would work well. In a warring kingdom forget about forts and run TG and GS instead.