Do you really need a mass gainer to bulk up?

Short answer

No. A gainer only adds calories, and adding 50 g of carbs to a protein shake did not grow more muscle. You just need to avoid running short on calories; eating far more only added fat. It is a tool for when you can't get enough calories from food.

Do you really need a mass gainer to bulk up?
'Gainer → more weight → more muscle' — does this formula hold?
A common belief: if you're skinny and want muscle, you need a gainer — protein loaded with carbs. We checked whether gainers actually build more muscle, and what the extra body weight really is.
What a gainer does is add calories
The high-calorie gainer group gained fat
28 people who lift took either a carb powder, a high-calorie gainer or a different nutrition supplement. The gainer group gained more weight, but fat mass and body fat percentage rose with it. The group that gained more lean mass (muscle and everything except fat) was the other supplement group, not the gainer group.
You still need enough calories
Running 500 kcal short a day stopped muscle gain
An analysis pooled randomized trials where people lifted while eating less than they burned for more than 3 weeks. Those short on calories gained less lean mass. At about 500 kcal short per day, lean mass did not increase at all. Strength gains did not differ.
So is more food always better?
The extra food turned into fat
39 people lifted for 8 to 12 weeks, one group eating 3,585 kcal a day and the other 2,964 kcal. The bigger eaters' fat mass rose 15%, versus 3%. Lean mass gains did not differ. In a 12-week trial of 23 people, the group that ate more gained 4.36 kg of fat versus 2.00 kg, with lean mass gains of 1.38 kg versus 1.34 kg.
What if you add carbs to a protein shake?
Turning it into a gainer made no muscle difference
20 people took two shakes for 8 weeks each: 30 g of protein, or the same shake with 50 g of carbs added. During the carb phase they ate 485 kcal more per day, but muscle size and strength did not differ. In a lab study of 10 people, adding carbs to enough protein did not raise muscle building either.
So when is a gainer useful?
Calories you drink fill you up less
15 people added 450 kcal a day for 4 weeks, as jelly beans or as soda. With jelly beans they ate that much less at meals, but with soda they didn't, so their weight went up. For someone with a small appetite who can't eat enough food, drinking calories is an easy way to add them.
Bottom line: a gainer is a tool, not a must
What matters for muscle is not running short on calories
A gainer just adds calories; adding 50 g of carbs to protein did not grow more muscle. Running 500 kcal short a day stopped lean mass gain, and eating far more only added fat. If food covers your calories and protein, you can skip the gainer, and use one when food alone isn't enough.