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.
Edit -
Nutrient paste probably wouldn't save much in the weight. The issue is the fact vegetation is the main food source. A nutrition bar with lots of calories probably requires foods they may not be able to handle. I found an article on zutrition.com listing 8000 calories as the daily requirement for a adult male gorilla. That is three to four times what you would feed a human soldier.
The issue here is the only advantage gorillas have is strength. Strength is not a big plus in the modern military. The only thing I might use them for would be to carry equipment for the humans, and I doubt they could actually keep up with the humans. I don't think they could stand upright to shoot for long. Humans can and do move and shoot, especially in close combat situations (Think SWAT teams storming a building, they move forward with the weapon in the firing position). Equipment and training are the important factors in modern combat. If you look at many of the battles that have happened in the last 25 years, highly trained troops were able to attain kill ratios above 10 to 1. The Blackhawk down battle is an excellent example.