Pho ga is about 50 calories lighter than pho bo, and the difference is almost entirely the meat. Chicken breast is 110 calories per 80g against 160 for the same weight of beef. The noodles are identical at 220 for 200g, and chicken broth is a little leaner than beef broth, around 45 for the bowl. So you are looking at 400 for a normal quan portion.
The trap is the Hanoi style bowl. Many quan use ga ta, free range chicken, served with skin on and chopped into bone-in pieces. The skin on an 80g portion adds 60 to 80 calories. Then there is the spoonful of fried shallots in oil that goes on top, another 40. Ask for a lot of chicken and you often get 120g instead of 80, which is fine for protein but now the bowl is 480.
Pho ga tron, the dry version tossed in soy, chili oil and fried shallots with the broth on the side, is the one to watch. The dressing is 80 to 100 calories of oil and the noodle portion is usually bigger because there is no broth to fill the bowl. Expect 500 to 550 for the same price.