You have to eat more, to gain weight. I don't know about powders.
If you can afford it then eat plates and plates of food at restaurants.. If it's not too bland for you and you are ok with putting the time in then you can make large amounts of food yourself and eat that.
You should at least try going nuts with eating, If you're not shocking the people around you with how much you eat, and out-eating all of them including the fattest, then you're not eating that much and you should be able to up the amount you eat without too much difficulty.
If you are out-eating anybody you know and shocking people at any restaurant you go to, and shocking anybody you eat with, from the sheer quantity that you eat, and not gaining the weight you want, then, under certain other conditions, such as liking variety or having a tight budget on how much you can spend at restaurants, then it's going to be hard to do more, though it's not clear whether you are like that. There are bodybuilding forums where people do succeed in gaining weight.
One solution i've heard of, though not implemented, is to hire a chef for a few hours to make you all your meals for 2 weeks, and put them in the freezer. People really dedicated to particular programs will eat pre-prepared meals. And they can carry a special bodybuilding bag with all their meals. I know an organisation not far from me that has a deal where you pay like £900 a month like $30 a day, and they'll make all your food fresh and you pick it up every day. Many people will pay almost that on food anyway so from that perspective they argue that it's not unreasonable.
So many of the personal trainers at gyms i've spoken to, when i've really probed them with questions, do not eat that well themselves. Like one guy was totally honest with me and said his mum makes him his food.. Another guy ate uncle bens rice.. Most people can't afford to eat plates and plates at restaurants(healthy or unhealthy), every day multiple times a day, and don't enjoy cooking.. And really even people in the industry are struggling a lot with the problem of eating as much as they should, despite the fact that they advise people on how to. e.g. they often eat fairly healthily but not as much as they'd like to. Two I spoke to that at one time had pre-prepared meals, I spoke to some time later and they hadn't kept it up because they wanted more variety. As one said to me, it's a battle, and they're all trying hard to solve it.