Australia triumphed by 53 runs over Sri Lanka on the Duckworth/Lewis method at Kensington Oval in a game ruined as a spectacle by the weather and overshadowed by confusion at the end.
Gilchrist's innings, the highest in a final surpassing the 140 not out made by Australia captain Ricky Ponting against India in Johannesburg four years ago, was the centrepiece of the champions' 281 for four.
But the game ended in farce with Australia thinking the final was finished due to bad light after 33 overs of the Sri Lankan innings, in a match reduced by rain to 38 a side, only for play to resume in pitch darkness on a ground without floodlights.
In all, Gilchrist, dropped on 31, faced just 104 balls with eight sixes and 13 fours as Australia set a daunting target in an innings reduced to 38 overs because of rain.
Victory meant Australia had won their fourth World Cup, improving their own record, in the tournament's nine-edition history.
In the process they extended their unbeaten run of games in the competition to 29 with 23 straight wins after their semi-final tie against South Africa in 1999.
Sri Lanka, in reply, finished on 215 for eight after a second rain break reduced their target to 269 off 36 overs.
They had a glimmer of hope during a second-wicket stand of 117 between left-handers Sanath Jayasuriya (63) and wicket-keeper Kumar Sangakkara (54).
The 1996 champions, who beat Australia in the final 11 years ago, lost Upul Tharanga early when he edged left-arm quick Nathan Bracken to wicket-keeper Gilchrist, Sri Lanka seven for one.
Jayasuriya, one of three survivors from the 1996 team, was reprieved on eight when a trademark square cut off fast bowler Shaun Tait was dropped by Shane Watson, diving forward, at third man.
At 36 for one, Glenn McGrath came on in his last match before retirement, the 37-year-old quick already this tournament's leading wicket-taker with a record edition haul of 25 wickets and the most successful World Cup bowler of all-time.
But keeper Kumar Sangakkara hit him off his usual good length for six over mid-wicket. Next ball the left-hander cover drove for four and the following delivery was disdainfully pulled to the boundary.
Jayasuriya then completed a 51-ball fifty before Sangakkara reached the landmark at a run-a-ball.
But spinner Brad Hogg delivered again for Australia when Sangakkara, on 54, pulled a long hop straight to Ponting at mid-on to leave Sri Lanka 123 for two in the 20th over.
And that became 145 for three in the 23rd when Jayasuriya was bowled for 63, trying to slog part-time spinner Michael Clarke.
Despite a fresh rain burst, the umpires kept the players on the field until, with Sri Lanka 149 for three in the 25th over, they called a halt.
Play soon re-started amidst confusion before the revised target was confirmed and then Sri Lanka captain Mahela Jayawardene was lbw to medium-pacer Watson.
Now, in gathering gloom, the game was up for Sri Lanka, Gilchrist's diving catch on the run to dismiss Russel Arnold giving Glenn McGrath a farewell wicket in his final match.
Earlier, Gilchrist and fellow left-hander Matthew Hayden's stand of 172 was a new first-wicket record for a World Cup final, surpassing the 129 shared by England's Mike Brearley and Geoff Boycott during West Indies' 92-run win at Lord's in 1979.
Left-arm quick Chaminda Vaas saw his now maximum eight overs cost 54 runs after Ponting won the toss.
Lasith Malinga took two for 49 in his eight overs after his first four cost just six runs.
Crucially first-change Dilhara Fernando, retained despite conceding 45 runs in five overs during Tuesday's 81-run semi-final win against New Zealand, dropped a low caught and bowled chance in his second over off Gilchrist's checked drive.
It didn't get much better for Fernando, his eight overs going for a hugely expensive 74, including Gilchrist's two fours and a six off successive balls. Gilchrist's fifty was up in 43 balls with two sixes and five fours and he was on the way to passing his previous best score this tournament of 59 not out against Bangladesh.
Perhaps his best shot was a six off Muttiah Muralitharan that soared over mid-wicket, the Sri Lanka star's seven wicketless overs costing 44 runs after he'd dismissed 23 batsmen in the tournament - second only to McGrath's pre-final haul.
Malinga's first ball back was smashed for six by Hayden.
Gilchrist then also struck Malinga to the long-off boundary to bring up a superb century in 72 balls with six sixes and eight fours.
Hayden, renowned as a power-hitter and the tournament's leading run-scorer, made a relatively subdued 38 before falling to Malinga's fifth ball back when driving to a leaping Jayawardene at extra-cover.
Gilchrist broke Ponting's record, with his skipper at the crease, when he drove left-arm spinner Jayasuriya for six.
Fernando eventually dismissed Gilchrist, skying to Chamara Silva running round to mid-wicket, but the damage had been done.