The left-hander hit an unbeaten 104 for his 13th century and Shivnarine Chanderpaul made 52 not out as Brian Lara's team made a mockery of Bangladesh's total of 161, winning with 13.2 overs to spare.
The West Indies clash with Sri Lanka in the last league match on October 14 at Mumbai to decide the qualifying-round winners, while winless Bangladesh and Zimbabwe meet in an insignificant game here on Friday.
Fast bowlers Corey Collymore (2-36) and off-spinner Marlon Samuels (2-27) also played key roles in setting up the West Indies victory before Gayle and Chanderpaul completed the job under lights.
The Bangladeshi seamers and spinners could never test left-handers Gayle and Chanderpaul who executed handsome shots during 164-run stand for the unfinished opening wicket.
Gayle went for big shots after reaching his half-century as he hit three towering sixes, the last landing on the roof of the stand. His 118-ball knock also contained 11 fours.
Chanderpaul provided valuable support, hitting six fours in his 104-ball knock.
Electing to bat, Bangladesh were strongly placed at 95-1 before floundering against the pace-spin combination on a batting pitch. They lost their last nine wickets for just 66 runs.
Aftab Ahmed, 20, sustained Bangladesh's hopes of posting a respectable total with a 55-ball 59 before the West Indies struck back through Collymore and Samuels in middle overs.
The duo shared four wickets in the space of 12 runs to reduce Bangladesh to 107-5, neutralising Ahmed's brilliant effort in the process.
All-rounder Dwayne Bravo then dashed Bangladesh's hopes of a recovery with a career-best haul 3-14 off 10 tidy overs.
Ahmed, dropped on 15 by Samuels off paceman Jerome Taylor, put on 85 for the second wicket with opener Shahriar Nafees (38). He hit two sixes and seven fours in his seventh half-century.
Wicket-keeper Khaled Mashud (22 not out) was the other batsman to reach double-figures in an inconsistent Bangladeshi batting performance.
The Bangladeshi middle-order batting wobbled, with Saqibul Hasan and skipper Habibul Bashar falling for ducks off successive deliveries from Collymore and Farhad Reza contributing just seven.
Bashar has yet to score in two matches of the ongoing tournament. He had lasted just two balls during his team's defeat in the previous game against Sri Lanka at Mohali.