Who were the the gods of nineveh?

That's exactly what happened back then. The Nineveh's people were worshiping a couple of gods, but the greatest of them had the figure of a standing man with a fish head.

God never did and will never do things randomly. He chose a great fish to swallow Jonah for a reason: the Nineveh' people thought Jonah was sent by their greatest god (the standing man with a fish head) therefore they listened to Jonah's message and feared the Lord. From the king to the last person in that city (the greatest in the world at that time), everyone went before God, the real God, to repent and turn away from their sinfulness ways of living.

God was offering a second chance to that city, and they took it.

But also God knew they won't listen to His messenger (Jonah, in this case) unless he will come in an amazing way related to their pagan belief. God used their religious system to help them turn to Him, the real God Almighty.

The Book of Jonah is all about God giving us a second chance to come back to Him and live for Him.