At a wedding reception, a man pointed to the groom and described him as the brother of the son of his grandfather's son. How is the groom related to that man?

Let's break down the relation from the man's perspective step-by-step:
1. "Grandfather's son" $\to$ The man's father (or uncle).
2. "Son of his grandfather's son" $\to$ The son of the man's father, which means the man himself or his brother.
3. "Brother of the son of his grandfather's son" $\to$ The brother of that son, which translates to the man's brother.

Therefore, the groom is the man's brother.