if you're working out hard, creatine pre-workout is worth it once you have reached a "wall" IMO. In a short 4 month deployment I spent over $700 in supplements and put on 24lbs of muscle, creatine and weight gaining protein were the only supplements worth the money. Creatine will give you 1 to 3 more reps once you feel like you can't possibly do another, it really does work and therefore will help add muscle.
However, I don't believe that is the best way to gain weight...if you can afford to spend the cash per month on weight gaining protein it works awesome..otherwise EAT, all the time. It actually becomes work you have to eat so often, im not talking stuffing yourself, but you need to be eating 6 meals a day and never let yourself feel hungry, if you miss a meal, you're backtracking...I've been there I know. Im 6'1" 185lbs and have decided to be happy with that as I just dont have the time to stay with a program to gain weight....I have gotten up to 210lbs, and got too busy to stick with it and was down to 185 in about 45 days.
you can do it if you really want to gain, but it never ends if you're a hard gainer, good luck.