Guardian Shield could use a nice upgrade to buff it up, maybe? Perhaps boost its range? Or allow overlapping shields to stack to a degree? Maybe a diminishing return system that starts at +1, then +.5, then +.25... something to give them an edge for having extra Sentries and giving enemy armies a better reason to take them out. If a Protoss Gateway Ball has -4 to all received ranged damage, it would be pretty important to kill.
Whatever the case, a Guardian Shield buff might make Gateway play a bit more viable.