Chasing 406 for victory at the Barsapara Cricket Stadium, India reached 406 for two in just 43.3 overs to take an unassailable 2–0 lead in the three-match series.
Gill was unbeaten on 223 from 133 balls, smashing 26 fours and eight sixes. His innings was the fastest double century in men's ODI history and the highest individual score by a captain in an ODI chase.
Rohit Sharma provided the perfect support at the other end, scoring 101 from 75 deliveries, including 10 fours and six sixes. The pair put on a remarkable 255-run opening partnership before Rohit was dismissed by Jayden Seales shortly after reaching his century.
Virat Kohli contributed 29 from 19 balls before falling to Shamar Joseph, while Ruturaj Gaikwad finished unbeaten on 31 as India completed the chase with 39 balls to spare.
West Indies had earlier posted 405 for seven, their first-ever 400-plus ODI total. John Campbell, Shai Hope and Amir Jangoo all scored centuries, with Campbell making 101 from 65 balls, Hope 104 from 94 and Jangoo a rapid 114 from 77 deliveries.
The match therefore became only the second time in men's ODI history that a team successfully chased a target of 400 or more. India's 406 was also their highest successful ODI chase, surpassing their previous record of 360 against Australia in Jaipur in 2013.
Gill's extraordinary innings capped a remarkable batting display and secured the series for India with one match still to play.