I can't say precisely with simplefit; though in my experience, as long as you eat a LOT of protein, say 1g/pound of body weight, plus a bit more calories than your body normally needs, and if you exercise consistently, you will gain muscle. Gained 20 pounds that way in the last 2 years and believe me it's tough for me!
EDIT : I feel it important to say that I primarily lifted weights during the time I gained that weight. However, I don't see any reason why simplefit wouldn't work just as well. It might be just a little slower, but the strength and endurance is all functional; you'll notice a difference in your everyday movements much more than with dumbbells strength workouts IMO.
Nutrition is 80% of the results and exercice 20%, but both need to be there for results to happen!