What gift did Athena give to the people of the city of Athens?

Athena gave the Athenians an Olive tree. Poseidon gave them a salt water spring.

olives were more valuable and so the Athenians chose Athena as their patron god.

To the person above, Poseidon did not give them a saltwater spring, he gave them the first horse

Actually, to the person above Poseidon gave the people a fountain not a horse... The person above you is right.