In a run-filled second match of the three-game series on Wednesday, Gill, the world's top-ranked ODI batter, remained unbeaten on 223 off 133 deliveries and brought up the quickest ODI century of his career before converting it into the fastest ODI double hundred.
The 27-year-old had scored his first ODI double-century against New Zealand in Hyderabad in 2023.
"When it was 13 overs, it felt like we had fielded for 25 overs, the way they were batting. Great game. At the half time, 400 on the board, there was a bit of pressure.
"No matter the pitch or ground, chasing 400 is not easy. Very proud of that performance and happy to stay unbeaten 'til the end," Gill told reporters.
Rohit Sharma set the tone with a 72-ball century, passing 12,000 ODI runs during a blistering 255-run opening stand with Gill, before Jayden Seales ended his innings on 101 shortly after he brought up his 35th ODI hundred.
Virat Kohli made 29 off 19 deliveries, and Ruturaj Gaikwad chipped in with 31 off 37 balls and with the Guwahati surface offering little assistance to the bowlers, Gill guided India to their target with 39 balls to spare.
Earlier, West Indies had looked in complete control after posting their first 400-plus ODI total, with John Campbell, Shai Hope and Amir Jangoo all scoring centuries.
Three batters making hundreds in the same ODI innings -- only the sixth time this has happened -- helped the touring side amass 7-405.
After Keacy Carty's early dismissal, Campbell blasted a 65-ball century, the fifth-fastest by a West Indian batter, racing to 101 with nine fours and five sixes before falling to Gurnoor Brar.
Captain Hope brought up his 20th ODI century and Jangoo hammered 114 off 77 balls.
Roston Chase maintained the momentum with a late cameo as the visitors plundered 76 runs from the final 10 overs.
