No, they wouldn't be more effective.

  The biggest issue would be the massive amount of food they would need.  A quick search quoted a male can eat around 50 to 75lb of food a day.  A typical human soldier can be expected to consume 4500 to 5000 calories per day.  This is slightly more than double the calories that a civilian would consume, leading me to expect a gorilla soldier would need to carry minimum of about 100lbs of food for each day.  Even at base the amount of food needed would make a logistical nightmare that could easily be attacked.

Another issue would be endurance.  Being much lighter we would likely be able to outlast the gorilla.

Obviously you wouldn't want to go hand to hand with them, but that doesn't happen often.