The loss for South Korea also ended its hopes of a semifinal place and the 2006 Asian Games champion will now play Oman in the playoff match for fifth and sixth place on Saturday.
India, which went into the match after playing out a goalless draw against Asian Games silver medallist Malaysia on Tuesday, got off to a quick start as Harmanpreet hit a crisp drag-flick in the fifth-minute off a penalty corner.
Five minutes later it was Gurjant who made it 2-0 with a field goal as India ended the first quarter with a 2-0 cushion.
South Korea's captain Lee Seung-Il reduced the lead with a goal in the second quarter and both the teams failed to find any goal in the third quarter.
A late blitz from India had Harmanpreet complete his treble with two rasping shots in the 47th and a minute before the hooter to become the tournament's third player to have a triple to his credit.
His teammate, Dilpreet Singh, and Pakistan's Aleem Bilal are the other two players to have scored three goals in a match. India now enjoys two days rest before meeting the fourth-placed team in the semifinals on Saturday.