After searching for a while, I found the following, TeamLava's Guide to Gifting, to be most useful as it explains how gifting (sending & receiving Gold & Mystic Maps) works in Dragon Story (and 'gifting' other TeamLava games).
Also useful to note that the 'Gifting Counter' is reset at 12:00 am PT (Pacific Time), every day.
Regarding the limit of receiving 20 Mystic Maps per day, I believe the true limit is 21, as explained here Is there a limit on Mystic Maps?
since I regularly receive 21 Mystic Maps per day.