Question About Weight Gaining Using a Supplement

Anyways one question I have is how exactly does this weight gainer stuff work? I know people say not to worry about calories but calories is still used when talking about maintaing ur weight and what not. My question is, if a 160 pound person requires atleast 2000 calories to maintain their current weight, wouldnt I after say a week gain like 10-20 pounds if I took that stuff everyday + my regular diet?

I only weigh 115 pounds and lately been eating roughly 1600 calories worth of food. Now with this extra 1100 calories from the weight gainer that would be over 2000 calories, so wouldnt I gain an extreme amount of weight in a week?

Please if you could explain this to me correct that would be awsome heh, I highly doubt it would put me up 10-20 pounds in a week. But if it does that is awsome!