Boomer's right, though his post made me laugh, mostly because it rings true. That said, I haven't had much success putting on weight, but then, I never really needed to since at around 5'10" and 180 lbs., I was rather normal. I've been working out for about two years and using whey protein shakes as supplementary intake in those years and, along with refining my diet to eating every 3 hours and just trimming as much as negative foods as I can, I'm only at 187, which is where I've been for months. The GNC protein shakes will take some adjusting; after a few weeks, your stomach shouldn't harass you as much, though I'll warn you ahead of time: Whey protein + high fiber intake + espresso = Chaos.
If you want to gain weight, you're really going to have crank up the eating, because a large part of the reason I never gained weight was because I simply was burning more than I was taking in, though I felt like if I stuffed myself with anything else, I was going to blow up. Trying to effectively double your eating habits and doing so while eating healthy, requires _a_lot_ of food. At your height and weight, you really do need to throw on some weight, particularly for grappling; most average guys will probably have a good chunk of weight on you that'll make things difficult for sure. Good luck.