Pathak, 58, died while he was swimming, officials of the Indian Embassy in Bangkok told PTI.
He had gone to Thailand with his family for a vacation. He is survived by his wife, a son and a daughter.
Pathak's body was sent to Krabi town, 60 km from Phuket. Indian Ambassador Anil Wadhwa said the mission is extending all assistance to Pathak's family.
An IPS officer of the 1977 batch, Pathak, who hails from Uttar Pradesh, was appointed Director General of Police of Gujarat on February 27.
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"In Amitabh Pathak, Gujarat has lost a dedicated officer. The state will always remember his services. Spoke to his son and offered condolences," Modi said in a post on Twitter.
On Independence Day, Pathak fainted while standing near the podium where the Chief Minister was delivering his speech at Bhuj in Gujarat. At the time, he said he suddenly felt fatigued and fell down.
In Ahmedabad, Home Secretary M D Antani told PTI that Pathak died while he was on vacation with his family in Thailand.
Before being appointed the DGP, Pathak was director of the Anti-Corruption Bureau. He became the state police chief after Chitranjan Singh retired.