26 year, 57kg with 5'6" height. it seems perfectly fine to me as per your age and no medical conditions are mentioned. The body started gaining weight with age as metabolism gets down. If you still want to gain weight I would suggest for a proper body assessment first.

Hello sir, this is Dr lavanya from My physiofix clinic As you mentioned that you are going for gym and want to gain weight ill just give my advice which may help you First: Go for "fitness evaluation" it will include body composition, BMI, Body fat, flexibility testing, muscular strength testing, muscular endurance testing, core endurance assessment, cardiovascular assessment, muscle power assessment, balance, posture analysis(according to this your gym trainer will form a proper plan and you can concentrate on particular lacking points that will benifit for you body building)

Go with plan 1.

Go with the plan1,but remember to increase weight of dumble in every third day.

For weight gain you can follow any of the program you mentioned above along with proper balanced diet plan with consistency and add cardio and weight lifting First do cardio followed by weight lifting for gaining

Plan 1 is best go for it.

Plan 1 is good but you need to consult a nutritionist too for proper amount of calories to be taken

For strengthening you can do gym and weight gain best option consult with a dietician.