What should I do with the piece of gold I obtained from a mixture of work and gambling money before I embraced Islam?

Your entry into Islam erases the sins that preceded it, as stated by Ibn al-Athir and Ibn Battal. This is supported by the Almighty's saying: "Say to those who have disbelieved, if they cease, what has passed will be forgiven for them," and His saying: "So whoever receives an admonition from his Lord and ceases, then what has passed is his." This is also supported by the prophetic Hadith: "If a servant embraces Islam and his Islam is good, Allah will expiate all the evil deeds he had committed previously." Ibn Rushd pointed out that if disbelievers were to embrace Islam, they would retain what they possess from usury, and the price of wine and pork. It has not been narrated that the Prophet, peace and blessings be upon him, commanded anyone who embraced Islam to dispose of what they had acquired through unlawful means before their conversion.