What attribute should the gift of Night confer if they choose to use it to raise their attributes?
Neither the Word of Night nor any of its gifts raise Attributes by default. If you were asking about perhaps buying Excellence of the [Night] (p. 29), the answer is "anything they can justify." I can think of ways to justify raising any Attribute with Night.
You mean by purchasing the universal gift that raises a stat to 18? If so, it can raise any of them.