Man of the Match Gayle made a superb 133 off just 135 balls and, along with Shivnarine Chanderpaul, added 154 runs in just 26 overs as the Windies overhauled the 258-run target with six overs to spare.
The South African effort with the bat looked competitive in the first sight, and given the clinical finishing of Pakistan by the Protea bowlers in the previous match, the target looked imposing.
But the West Indians had other ideas and Gayle took no time to make the intentions clear.
He blasted Shaun Pollack for two straight drives, one better than the other, in the very first over and the veteran Protea bowler looked like a kindergarten kid chancing his arms. That was not the end for Pollock and the bowler was subjected to further humiliation when he was hoisted over the long off for Gayle's first six. Graeme Smith took Pollock off the attack and introduced himself in the 11th over. Gayle, however, made no concession, reverse swept Smith for four and followed it up with a cracking straight drive towards the fence. He completed his fifty in just 48 balls.
At the other end, Chanderpaul was playing the perfect foil matching Gayle shot-for-shot. He pulled Makhaya Ntini through midwicket for four in the fourth over to join the party. Chanderpaul held all the bowlers in same esteem, smashing Pollock straight back for a boundary and then welcoming Andre Nel with a pull to fence.
His best shot, in fact the shot of the match, came in 10th over when he flicked Nel over fine leg boundary for a six. The ball just kissed his bat and sailed over in to the stands.
The duo added 75 runs in just 10 overs as the West Indian 100 came in 17th over.
Chanderpaul, suffering from cramps,' took a runner and reached his 42nd fifty in 50 balls. However, he could not carry on for long and had to retire hurt for 57 (70 balls, 4x4, 1x6) Meanwhile, Gayle was unperturbed and continued to murder the South African bowling. The southpaw hammered Jacques Kallis for a six over midwicket in the 18th over and stepped down the wicket to young spinner Robin Peterson and ball could have lost in the space only it fell short of escape velocity.
Gayle tapped Ntini for a single to complete his 15th hundred -- his third in the tourney -- of just 106 balls. Ramnaresh Sarwan, on his part, chipped in with a quick fire 27 (27 balls, 5x4) and blasted Ntini for back-to-back fours in the 34th over but the bowler had the last laugh when he found Sarwan plumb in front of the wicket. West Indies were 196 for one.
Dwayne Bravo (15), Brian Lara (9) and Runako Morton (0) departed in quick succession but it came too little, too late for the Proteas and Gayle stayed there till the end to finish it off. He hit a cracking straight drive off Peterson to take the Windies home.
Marlon Samuels remained unbeaten on five.
For South African bowlers there was little to cheer and Graeme Smith, with one for 43 off his 10 overs, was the most successful of the lot.
Earlier, Herschelle Gibbs silenced his critics with a magnificent half-century (77) and powered South Africa to 258 for eight. Gibbs, who had an unimpressive outing so far accumulating only 16 runs in three matches, batted with resolve and scored 77 (90b, 4x4, 1x6) as South Africa shrugged off a poor start to post their highest total in the tournament.
With the top guns misfiring again after South Africa elected to bat, Gibbs refrained from extravagant shots and relied more on rotating the strike. Meanwhile, AB de Villiers played the perfect foil to Gibbs and the duo added 92 runs in 108 balls for the fourth wicket when the team was struggling at 94 for three.
Gibbs finally fell in the last over when he tried to hit Bravo out of the ground but could not connect and holed it to Lara at the mid-wicket. His 77 came in 90 balls with four fours and a six.
Towards the end, Mark Boucher (16 off 11 balls) and Peterson (15 not out off 12 balls) also exhibited some stroke-making to push the Proteas to a challenging total.
It was after South African top-order batsmen threw away their wickets after getting good starts. Graeme Smith hit three boundaries off Ian Bradshaw before being castled by Jerome Taylor for 19.
Jacques Kallis added 38 runs with Loots Bosman before he tried to slash a widish delivery by Bravo in the 17th over but only managed to top-edge it to Sarwan at third man. He made 16 off 38 balls.
Loots Bosman looked in real good touch and appeared to justify his inclusion ahead of Boeta Dippenaar. After a cautious start, Loots smashed Taylor through midwicket in the fourth over to bring his first boundary. In the 12th over, he hit the same bowler though cover for four and then struck a couple of more boundaries off Dwayne Bravo in the next. When he seemed all set for a half century, he edged Marlon Samuels in the 23rd over and Gayle made no mistake at covers. Bosman fell for 39 (58balls, 6x4) with the scoreboard reading 96 for three.
For West Indies, Jerome Taylor, Dwayne Bravo and Marlon Samuels took two wickets each while Bradshaw picked one.
South Africa made two changes in their squad bringing in Loots Bosman and Robin Peterson for Boeta Dippenaar and Charl Langeveldt while West Indies also made a couple of changes inducting Dwayne Smith and Ian Bradshaw in place of Fidel Edwards and Correy Collymore.