No one can tell you what your results are going to be, because we don't know what your diet is like, how you're going to train, how hard you're going to do that training, how much rest you'll get and what your genetics are.
The fact that you want to know whether you'll have gained four stone in the next nine months makes it look like you're basing your decision to start training whether or not you'll be massive by summer, which doesnt speak volumes about potential for dedication.
The best thing for you to do is to do some reading. Get the sound basics of nutrition, and training down, try a program (NOT one from your gym) and start lifting! You'll soon find out whether or not its for you.
All we can say is that if you do things properly, you WILL grow.