Alot of supplements are worth their money. However before you get into supplements, make sure your diet is in line. If your a hard gainer then that means ALOT of food. Probably something like 2 g protein per lb and 4 gram carbs per lb. Lots of water. Some veggies or even fruits for fiber.
Now, assuming your diet is in line.. supplements that are worth their money=
1. Fish oil. If you can get one standardized for at least 50% epa/dha (like 500mg total epa/dha per 1000mg fish oil) that would be best. When to take= with meals, except post workout. Take 2-5 caps per meal.
2. Whey protein. Your best off getting an isolate. Preferably a hydrosylate. But regular old whey concentrate will work just fine as well. When to take= before/after your workout, and first thing in the morning.
3. Casein Protein. Your best off getting a casein with some whey and egg white too if you can. But any casein will work. When to take= before bed.
4. Extra vitamins= Vitamin C, Zinc, Magnesium, Calcium.
5. Evening primrose oil. Most people are already sufficient for omega-6 fats, but EPO is high in GLA, which most people DONT get enough of. Take this with your fish oil (2-5 caps per meal as well. Might not need as much EPO as fish oil. )
6. Green Tea extract. Standardized for at least 300mg egcg. Try to get 600-1200mg egcg per day though. Take this with fish oil/EPO.
7. Multivitamin. Animal packs are good, but pricey. Take this once a day, whenever.