Yes, GOD is a Scrabble word!

Letter values in Scrabble US: G 2, O 1, D 2.

Is it a valid word?

GOD is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the supernatural being conceived as the perfect and omnipotent and omniscient originator and ruler of the universe; the object of worship in monotheistic religions
any supernatural being worshipped as controlling some part of the world or some aspect of life or who is the personification of a force
a man of such superior qualities that he seems like a deity to other people
a material effigy that is worshipped

Also: supreme being, graven image.