is not described as a gift, Another word for Mahr in Arabic language is "Sadaq", "Sadaq" is derived from "Sidq", "Sidq" means truth, therefore Mahr means that man is serious to marry the woman.
The Mahr is an indication for the serious intension.
some definitions about Mahr:
The Mahr (Dowry)

The mahr (dowry) is something that is paid by the man to his wife. It is paid to the wife and to her only as an honor and a respect given to her and to show that he has a serious desire to marry her and is not simply entering into the marriage
here is the source:
http://www.islamswomen.com/marriage/fiqh_of_marriage_6.php