Gregory Papatheodoros was born in 1770 in the village of Albaina (Olympia) to the parents of Theodoros Papatheodoros and Anastasia. His early education began in his home village, but he pursued further studies at the Monastery of Vytina, which was widely known for its rich and diverse library.  As well as his academic interests, Gregory had a passion for languages. He was fluent in Turkish, Arabic, French, Italian and of course Greek. However, his passion and calling centred around his faith and community.

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At the age of 30, Gregory embraced a path of service in the Church. With steadfast faith and devotion, Gregory quickly climbed through the ranks of priesthood.  In 1800, he was ordained a Deacon and was later elevated to the position of Presbyter and Chancellor. His significant role as the Bishop of Methoni, Navarino, and Neokastro transpired between 1816-25, by Patriarch Cyril VI of Constantinople.

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In 1817, Gregory embarked on a crucial journey to Russia, where he fervently advocated for Greek independence before the tsar, sharing the harrowing tales of the oppressed Greeks under Turkish rule. Having successfully received Russian support, Gregory returned to Methoni where he later joined the Friendly Society (a secret political and revolutionary organization founded in 1814 whose sole purpose was to overthrow the Ottoman rule of Greece and establish an independent Greek State).  His growing reputation, admiration and respect saw him actively recruit more than 150 fellow patriots into the Friendly Society. This played a key role in the Greek Revolution of 1821, leading military actions that resulted in the successful sieges of Methoni and Neokastro.  With a defiant army, the Turkish forces were forced to hand over the keys to the castle of Methoni to the general commander, Bishop Gregory.