How can I apply for a Greek Golden Visa through my existing property?

Unfortunately not. Only if was bought after 2015.

According to the Greek Immigration Law currently in force, third country citizens who have full ownership and possession of real estate property in Greece, which they have purchased before the enactment of law 4146/2013, provided that they had purchased the real estate property for a minimum of €250,000 or the current objective value of their real estate property is at a minimum of €250,000, are entitled for Greek Golden Visa benefits. Please ask a Notary to provide the tax value of your property now. Furthermore, address your case to an experienced consultancy firm the soonest, in order to ensure the affection of the recently voted amendments.