How do i gain weight without a special diet?

See below: That doesn't make sense logically. Every diet has some purpose. If it's your native diet, then it may not lead to weight gain (increase muscle mass and not fat & sugar hopefully). In that case, an alternative diet would be "special" to you - like increased lean meats/fish. The goal is to increase mass while maintaining adequate intake of all other nutrients & minimizing cholesterol, fat, sugar.

Calories in: not everyone can. Fundamentally, it is calories in. But, for it to be in muscle it takes a lot of hard, specific work and months for significant effect. Find the maximum you can lift once, go with 90% of that weight for 6-10 reps, repeat at 2/3rds the number then 2/3 of that. 1 3-set a day. takes 1-2 months. For calories, sweets/fats and bite sized pieces.