Answer by taking a quote from the following article:

Adam Craig Gilchrist, AM (; born 14 November 1971), nicknamed "Gilly" or "Churchy", is a former Australian international cricketer and former captain of all formats. Widely regarded as the greatest wicket-keeper-batsman in the history of the game, Gilchrist held the world record for the most dismissals by a wicket-keeper in One Day International (ODI) cricket until it was surpassed by Kumar Sangakkara in 2015 and the most by an Australian in Test cricket. He was captain of Middlesex in the domestic arena. Gilchrist was selected for the state under-17 team, and in 1989 he was offered a scholarship by London-based Richmond Cricket Club, a scheme he now supports himself.

Cricket has for many years debated whether batsmen should "walk", that is to agree that they have been dismissed and leave the field of play without waiting for (or contrary to) an umpire's decision. Gilchrist reignited this debate by walking during a high-profile match, the 2003 World Cup semi-final against Sri Lanka, after the umpire ruled him to be not out. He has since proclaimed himself to be "a walker", or a batsman who will consistently walk, and has done so on numerous occasions. On one occasion against Bangladesh, Gilchrist walked but TV replays failed to suggest any contact between his bat and the ball. Without such contact, he could not have been caught out.  Gilchrist's actions have sparked debate amongst current and former players and umpires. Ricky Ponting has declared on several occasions that he is not a walker but will leave it to each player to decide whether they wish to walk or not. While no other Australian top order batsmen have expressly declared themselves to be walkers, lower-order batsmen Jason Gillespie and Michael Kasprowicz both walked during Test matches in India in 2004. In 2004, New Zealand captain Stephen Fleming accused Gilchrist of conducting a "walking crusade" when Craig McMillan refused to walk after Gilchrist had caught him off an edge from the bowling of Jason Gillespie in the First Test in Brisbane. After the appeal was turned down by the umpire, who did not hear the edge, Gilchrist goaded McMillan about the edge, and McMillan's angry response was picked up by the stump microphone: "...not everyone is walking, Gilly ... not everyone has to walk, mate...". The taunt was effective, however, as McMillan, perhaps distracted, missed the next ball and was given out leg before wicket. Gilchrist said in his autobiography that he had "zero support in the team" for his stance and that he felt that the topic made the dressing room uncomfortable. He added that he "felt isolated" and "silently accused of betraying the team. Implicitly I was made to feel selfish, as if I was walking for the sake of my own clean image, thereby making everyone else look dishonest."  Gilchrist has been noted for his emotional outbursts on the cricket field, and has been fined multiple times for dissent against umpiring decisions. In January 2006, he was fined 40% of his match fee in an ODI against South Africa. In another instance, in early 2004 in Sri Lanka, Gilchrist audibly argued with umpire Peter Manuel after batting partner Andrew Symonds was given out. After the argument concluded, Manuel consulted umpiring partner Billy Bowden and reversed his decision, recalling Symonds to the crease. Gilchrist was also reprimanded by the Australian Cricket Board for publicly questioning the legality of Muttiah Muralitharan's bowling action in 2002, as his comments were found to be in breach of the clause in the player code of conduct relating to "detrimental public comment".  During the 2003 World Cup, Gilchrist accused Pakistani wicketkeeper Rashid Latif of making a racist remark towards him while the latter was batting in their group match. Latif who was cleared by match referee Clive Lloyd, threatened to sue Gilchrist for this claim.

What happened in the 2003 world cup match?
after the umpire ruled him to be not out. He has since proclaimed himself to be "a walker",