It possibly means that the person addressed (that is, the person being spoken to) has some sort of great natural ability or talent. Such a natual ability or talent is called a "gift". If the person gives anything less than his best (that is, if he does not do the best he can), it would be as if he were sacrificing (destroying, giving up) his gift.

not sure it is not really a complete sentence or a saying I am familiar with. where did you hear this, I think some one mixed up a saying, like don't give less then your best, or give it all you have got, to do other wise you are cheating yourself.