Should I mention mental condition? I graduated on time and on the top back then, but after that I had some problem with major depression and anxiety, hence I have some gap on my CV and for a while, I either couldn't work or worked below my qualification. My world literally fell apart but now I am trying to get back to my old dream. Next year, if everything goes well, I want to go get a master degree. 

How should I explain myself? Do I need to mention my history with depression? I am from a country where mental health is still really badly stigmatised, but I'd like to apply abroad. I am not sure what's appropriate to say and what's not.
Thank you.
I am responding to this question as a senior academic from Australia, where discrimination on the grounds of a disability (https://www.humanrights.gov.au/employers/good-practice-good-business-factsheets/quick-guide-australian-discrimination-laws) is illegal.  Other countries will have different rules and protections.

I see no reason for you to list this in your CV. If a gap is uncovered by an assessor or interviewer, then they will put this to you and seek clarification. It is reasonable for them to ask and it is equally reasonable for you to simply state that the gap was due to personal reasons not material to the decision. Where I work, this would be an acceptable response and no more need be said about it. If they press (and I wouldn't), you may state that the gap was due to personal health issues. 

If they find out -- say, one of your referees inadvertently mentions it -- then the knowledge of your disability cannot be used as grounds for any decision that they will make.

If you get a spot as a Master's student, then part of the orientation program is a discussion of opportunities for people with disabilities. This is usually handled by another office independent of the academic department of which you are a student. In Australia, the nature of your disability and its management is protected by privacy legislation.

Good luck!