It's common to gain 5-10 lbs of body weight within a 3 week period using creatine. Creatine works by super-hydrating the muscle cells with water to the extent they are in a better environment both as to recovery, and training in general, allowing one to move more weight, with more intensity and less risk of injury. The extra water also allows for increased uptake of nutrients.
The initial increase in weight is all water, but with the added strength and size you will be able to lift more weight, which will add more muscle if everything else is in order. Creatine is not a miracle supp, just a good training aid.