Gangjee, who suffered an early setback in the form of a double bogey on the par-5 second hole, finished with a four-day total of 14-under 274 in tied seventh place.
Himmat Rai also suffered a double bogey though late in the round on par-3 15th but still finished with a 68 and moved to a creditable tied 13th at 12-under 276.
Sujjan Singh (67) made a good return from injury with a total of nine-under 279 alongside fellow Chandigarh youngster, Ajeetesh Sandhu (68) in tied 24th place. Shankar Das (69) finished in a tie for 39th at seven-under 281.
Perera plays on the Indian PGTI Tour also, where he has won in the recent past.
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Overnight leader Chawalit was tied with Mithun on 17-under heading into the last hole but the Thai set up a three-foot birdie putt with an exquisite approach shot at the par five 18th hole to win his fourth Asian Tour title at the Royal Mingalardon Golf and Country Club.
Chawalit closed with a three-under-par 69 while Mithun, aiming to be the first Sri Lankan to win on the Asian Tour, fought bravely with a sensational 65 at the USD 300,000 event, which is the curtain raiser for the 10th Asian Tour season.