General

I wanna start to add more calories throughout the day, and I started to eat more greasy foods and recently I started drinking weight gainer shakes.
Is it a bad idea to drink 1 gainer shake per day to add calories?

It depends on what’s in the gainer shake and how well your body handles it. If you’ve made the shake fairly nutritionally balanced (or if it’s balanced out well by the rest of your diet) and it’s just adding calories, it’s excellent. If it’s just very heavy on sugar and/or fat and not much else, it might make you sick if you try to drink it too long-term, especially if you’ve got preexisting stomach problems. In that case it would be better for one to three week gaining cycles with periods of normal diet in between.

I would not do this.

Gainer shakes are not just a way to get more calories. Depending on how you make them, they can be high in other things that, if consumed too often, can suppress your appetite and lead to malnutrition. And eating a diet high in greasy foods is not sustainable either.

You need to take stock of what you eat, how much you eat, and how many calories you burn. You need to eat a balanced diet, but with bigger portions. You can play around with the specifics as long as you hit your macro and micro nutrients. You can tweak how many calories you burn too - just make sure you maintain some level of fitness so you can stay fat.

If you cannot get enough calories normally, then I recommend weight gain shakes. I do not recommend daily shakes unless you are really struggling. Not only can that negatively impact your appetite and cause health complications depending on the shake - but it can be a lot more expensive in the long run.

I drink serious mass gainer shakes, one portion is around 1200 calories.
For now I don't get sick after drinking them, I take it at the end of the day almost before going to bed.