To think about just gaining weight,
I used to be 173 and 49kg.

Now after if you called it gaining weight (fats more likely at the tummy) I am now 53/54kg... though.... Well, it's time for some protein intake.. and some exercise

Have been trying the Horley's Weight Gainer.

Only use it when I am going to the gym later or after gym.
Surprisingly I used to weigh like 50kg just a year ago, but suddenly weight jump up to 58kg.

With a height of 171cm, I guess going up to 60kg should be the optimum