Gaining weight is not just to eat junk / sweets.
You should gain a healthy weight .
Try few things:
1. Small, freq meals
2. fruits , milk products minimum 2 times /d
3. Balanced diet comprising cerals, pulses , veggies , fruits and milk pro. everyday
4. preffer fruits like mango, banana, grapes.
If you can get back to me with your personal diet recall, we can set individualized diet plan