At what gate was Stephen stoned?

Answer:

"Then they cried out with a loud voice, stopped their ears, and ran at him with one accord; and they cast him out of the city and stoned him. And the witnesses laid down their clothes at the feet of a young man named Saul. And they stoned Stephen as he was calling on God and saying, "Lord Jesus, receive my spirit." Then he knelt down and cried out with a loud voice, "Lord, do not charge them with this sin." And when he had said this, he fell asleep" (Acts 7:57-60).

The biblical text does not say, so to make any claim would be mere speculation.