Should I mention mental condition?
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!