My experience is I only gain weight when I load (3-4 scoops a day). When I did that for 5 days I gained 10 lb of what was certainly 9 lb of water weight.
It does make you gain weight, but it is generally just water weight. It's not fat or anything. How much depends on the size you were to begin with I think.
If it's water gain, I wonder if it's water that the body wanted in the first place. I'm drinking over 100 oz a day at a body weight of 190 and don't notice a water gain. hmmm, I wonder if there is any scientific study on this.
It's been quite thoroughly studied. Look it up.