The Nephilim are mentioned only briefly
Genesis introduces the Nephilim before the flood and says they were also present afterward, but it gives few details about them. The verse does not provide a clear explanation of their ancestry, nor does it trace a family line into the later books of the Bible (Genesis 6:4).
They appear again in the report of the twelve spies sent into Canaan. The spies said they had seen the Nephilim and connected the Anakim with them. Yet these same spies were terrified by the land and discouraged Israel from trusting God's promise, so their report needs to be read in that setting (Numbers 13:31-33).
There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.
Goliath is called a Philistine from Gath
When Goliath enters the story, Scripture identifies him as a Philistine of Gath. It records his extraordinary height and the weight of his armour, showing why Israel's soldiers were shaken by him (1 Samuel 17:4-5).
But the account never uses the word Nephilim for Goliath. It does not name his father, his wider clan, or any descent from the Anakim. That matters. A possible connection is not the same as a statement Scripture actually makes.
And there went out a champion out of the camp of the Philistines, named Goliath, of Gath, whose height was six cubits and a span.
Why people connect him with giant clans
There is a real geographical link. After Joshua's campaigns, Anakim remained in Gaza, Gath, and Ashdod. Since Goliath came from Gath, he lived in a place associated with these unusually large Canaanite people (Joshua 11:22).
Later, David's men fought several formidable Philistine opponents linked with "the giant." Ishbi-benob and Saph are described as sons of the giant. The passage also tells of a man of great stature encountered at Gath, who was born to the giant and had six fingers on each hand and six toes on each foot (2 Samuel 21:16-20). These men show that Gath had an ongoing association with giant warriors, but the passage still does not label Goliath a Nephilim.
The story points beyond Goliath's size
It is natural to wonder who Goliath was and where his great size came from. Still, 1 Samuel 17 puts the spotlight elsewhere. David did not meet Goliath because he had solved every mystery about his background. He came trusting that the Lord was not limited by swords, spears, or human strength (1 Samuel 17:45-47).
So the careful answer is no one knows. Goliath may have belonged to a people connected with the giants of Gath, but the Bible does not call him a Nephilim. Let the unanswered detail stay unanswered, while taking seriously the passage's clearer message: God can deliver his people when the odds look impossible.
Then said David to the Philistine, Thou comest to me with a sword, and with a spear, and with a shield: but I come to thee in the name of the Lord of hosts, the God of the armies of Israel, whom thou hast defied.