For the career gap, the best way is to fill the gap in your resume with "learning" or "receiving some professional training" and "doing a part-time job", so as not to make you feel that you are very idle about HR.
Of course, if you write down your experience in volunteer work, you can also do it. The premise of this writing is related to the position you are applying for. If it's not relevant, just tell me briefly. But never make it up.