Introducing a woman, Namrata said, She is the daughter-in-law of the grandmother of my father's only son. How is the woman related to Namrata?

The woman is Namrata's mother.

Father's only son is Namrata's brother.

The grandmother of that son is Namrata's father's mother.

The daughter-in-law of that grandmother is the wife of her son, Namrata's father.

That wife is Namrata's mother.