It is okay to say that, but it is not the best way to express it. "Gone down" does not pack a very hard punch. It's better to say 'decreased' instead.
My weight has decreased by two kgs.
The reason decreased seems better is that it is more specific. 'Gone down' can refer to some people who have gone down to the basement, but the word 'decrease' is exactly right for your purpose.
Student or Learner