What is the group of food to eat in order to gain weight?

Eating calorically dense foods is what’s going to help you put on mass. Foods that are calorically dense are high in calories but don’t fill up your stomach very fast. Examples: cheeseburgers, quesadillas, milk, bread, peanut butter, bacon and eggs.

However, it depends if you want to build muscle or if you’re just looking to get fat. Especially because this depends on the food groups you want to focus on.

As others have mentioned, fat is the macro-nutrient that is the highest in calories. Therefore, eating a big meal from McDonald's will help with gaining weight but not necessarily good for building muscle, due to the negative effect fat has on protein synthesis.

Here’s a great video for a (relatively healthy) Homemade Mass Gainer that I used while struggling to gain weight: Calorie Calculator

Gaining healthy weight is much more useful for longevity and maintaining your metabolism. Examples of Healthy Medium-High Calorically Dense foods: Sweet Potatoes, Nuts, Avocados, Quinoa, Tahini and Fruit.