Priyanka is married to Dileep's father. Gayatri is the daughter of Nitesh. Ketan is the son of Gayatri's only brother, Dileep. How is Ketan related to Nitesh?

Let's reconstruct the family relationships step-by-step:
"Ketan is the son of Gayatri's only brother, Dileep."

This implies Dileep is the brother of Gayatri.

Ketan is the son of Dileep.

"Gayatri is the daughter of Nitesh."

Since Gayatri and Dileep are siblings, Nitesh is the father (or parent) of both Gayatri and Dileep.

"Priyanka is married to Dileep's father."

This shows Priyanka is the mother of Dileep and Gayatri, making Nitesh the father.

Now determining the relation between Ketan and Nitesh:

Dileep is Nitesh's son.

Ketan is Dileep's son.

Therefore, Ketan is the son of Nitesh's son, which makes Ketan the Grandson of Nitesh.