In Capital letters^x, Genitives^x

1. Can somebody explain me why I can write "a child is a God's gift" while "the dog was a Peter's gift" is wrong ? Isn't "God" (if written with the capital letter) a proper noun as well? 2: Moreover, as for the undefined article before the saxone genitive, please what's the difference...

That was I, Believer.We must now distinguish between proper names and proper nouns. A proper noun is a single word like Milwaukee, Tuesday, Christmas. Proper nouns (I now quote from Quirk) often combine with descriptive words (descriptors), and which also begin with a capital letter, to make...