Bengal lad Lahiri came up with a three-under-69 in the final round of the PGTI's year-ending championship to end the week with a total of 17-under 271 to clinch his maiden professional title at the RCGC and 13th overall.
Gangjee, who is playing host to Lahiri at his Alipore residence during the tournament, finished runner-up at 13-under-275.
Overnight leader by three strokes, the three-time Asian Tour winner had an early hiccup in the final round as he double-bogeyed the second after his second shot ricocheted off a water sprinkler.
In fact, the birdie on the ninth hole, where Gangjee bogeyed was the turning point of the game as Lahiri increased the lead.
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Lahiri's putter was on fire as he sank birdie putts from 15 to 20 feet on the fifth, ninth, 16th and 17th. He also landed his approach within two feet for a tap-in birdie on the 12th.
Lahiri's only error on the back-nine was the bogey on the closing 18th as he missed on to finish with a five stroke win.
He had a relatively quieter back-nine with two birdies and a bogey.
Gangjee was a threat to Lahiri in the initial stages of the round as he made some long putts on the first, second and fifth.
However, bogeys on the eighth, ninth and 12th pulled him back and he ended up with a three-under-69 for the day.
Abhinav Lohan of Faridabad and Delhi's Chiragh Kumar finished tied third at 10-under-278 while Jyoti Randhawa was a further shot behind in fifth place.
Gaganjeet Bhullar and Chikkarangappa of Bangalore were in tied eighth at six-under-282.