what does "by God"mean?
His mother took the milk-gourd without comment on the quantity. She strained the milk and poured a cupful and took it to the sick room. He followed her. Penny was awake. He smiled weakly.
He whispered thickly, "Ol' Death got to wait a while on me."
Doc said, "You belong to be kin to the rattlesnakes, man. How you done it without whiskey, I don't know."
Penny whispered, "Why, Doc, I'm a king snake. You know a rattler
cain't kill a king snake."
Buck and Mill-wheel came into the room. They grinned.
Buck said, "You ain't purty, Penny, but by God, you're alive."

Question

what does "by God"mean?
His mother took the milk-gourd without comment on the quantity. She strained the milk and poured a cupful and took it to the sick room. He followed her. Penny was awake. He smiled weakly. 
He whispered thickly, "Ol' Death got to wait a while on me." 
Doc said, "You belong to be kin to the rattlesnakes, man. How you done it without whiskey, I don't know." 
Penny whispered, "Why, Doc, I'm a king snake. You know a rattler 
cain't kill a king snake." 
Buck and Mill-wheel came into the room. They grinned. 
Buck said, "You ain't purty, Penny, but by God, you're alive."

Answer

It simply serves to emphasise the next phrase. It has nothing literally to do with God.

Answer

It simply means that by the grace of God, or by a quirk of nature. So, the writer is saying that they expected the person to be dead, but he'd survived another night thanks to a force the writer is calling God.