A) God bless you, or May God bless you. This is a complete sentence. 'May' is rarely used except by a priest delivering a blessing.
B) If God
'sis willing. The expression is normally just 'God willing'. It can go at the beginning or end of a sentence.
C) So help me God. This can go at the beginning or end of a sentence
D) For God's sake. This can go at the beginning or end. It sometimes stands alone as an expression of frustration.
The first two are rarely used in BrE these days except by older Christians. The third is not common at all. The fourth is quite commonly used by non-Christians as an expression of frustration.