What is the best product from GNC for gaining mass, muscle, and strengh, closest to a steriod?

What you are talking about is a costume called "muscle suit".

When you get serious, there is nothing but to learn the business. Study books about nutrition so you know how to eat right. Study books about anatomy so you know each muscle and how to exercise it. If you want more than that, you need professional guidance.

www.amazon.com/Adelle-Davis/e/B001J3RULO
www.adelledavis.org/
cronometer.com/ (I haven't tried the software.)
charlesatlas.com/ An exercise plan that has been successful for over 80 years and requires no equipment