If your father's father (your grandpa) was born in Malta before 1 September 1964, then your father Is automatically a Maltese citizen. You do not automatically acquire Maltese citizenship from him; but you can apply for citizenship by registration on the basis that your father is a Maltese citizen as were your grandparents and great grandparents. To pursue this, you'd need to contact the Malta Citizenship Agency, get the application, gather all the documents that prove your ancestry, and submit your case.
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