First of I'd like to hear more about your current nutrition and how much exactly are you eating. Or do you even have any idea about how much you are currently eating.
I can't even tell how many times I've heard this "I eat like horse" sentence and unsurprisingly when we take a look at what the person is really eating, it's actually not that impressive.
Weight gainer would obviously help to add some extra calories but I think you are better off getting those calories from more nutrient dense sources.
The sad reality is that if you want to build muscle you need to learn how to eat. The same effort you put in at the gym should be put in the kitchen as well.
If you have hard time eating enough there are several ways and tricks to get around this, implementing more low-volume, nutritious and very calorie-dense foods like nuts for instance.