Bob, this is a "two hershey bar" question and one I've often wondered about, so I went to various sources to see if this is the excepted spelling for GOD.
NELSON'S NEW ILLUSTRATED BIBLE DICTIONARY gave me various names for GOD, Jehovah/Yahweh is another name for GOD. It is taken from the verb "to be" meaning simply, "HE IS". There is only one place in the Bible, Exodus 3:14, where the full name of GOD is found. It means "I am who I am" or "I will be who I will be".
Within my family are two ministers, both spell GOD, not G-D.
One thing I did notice in all of the reference books, not a single one spells GOD's name G-D, therefore I have come to the conclusion that the correct spelling for his name is GOD and that it is not a sin. If it isn't a sin, then how can they be absolved of something that doesn't exist?
I had a neighbor look at that designation and they thought it was the abreviation for a certain phrase that takes GOD's name in vain and to be perfectly candid, the first time that I saw it, I thought the same thing. This forum is the only place that I see this done. It makes me wonder...