Gambhir made his ODI debut against Bangladesh in the TVS Cup in 2003. In his third match, he scored 71 and was named Man of the Match. His maiden century (103 off 97 balls) came against Sri Lanka in 2005. In 2004, he made his Test debut against Australia in the fourth and last Test match of the Border Gavaskar Trophy but did himself no favours by getting out for 3 and 1. He made amends in his second Test, however, scoring 96 against the South Africans. His maiden Test century came against Bangladesh in December 2004. Gambhir then made a number of starts in the home series against Pakistan in 2005, but was able to make only one half-century in six innings. He made 97 in Zimbabwe later that year, but failed to reach 30 against Sri Lanka at home, repeatedly struggling against Chaminda Vaas, and was subsequently dropped from the Test team. He was replaced in Tests by Wasim Jaffer, who made a double hundred and a hundred in seven Tests.

While Gambhir was out of the Test team, he played a number of One Day Internationals for India between 2005 and 2007. However, he was not selected for the 2007 Cricket World Cup as the selectors opted for a top-order of Sourav Ganguly, Virender Sehwag, and Sachin Tendulkar. After India's first-round exit from the tournament, Gambhir was selected for the One Day International on India's 2007 tour of Bangladesh. Gambhir scored his second century on that tour and was subsequently selected for the One Day International on India's tour to Ireland in 2007. He scored an unbeaten 80 against Ireland in the first game of that tour and was awarded the man of the match award for that effort. In the post-match interview, he indicated that performing more consistently was a top priority for his career as he had done so in the past. [4]

Gambhir was selected in India's squad for the 2007 ICC World Twenty20, which India went on to win in South Africa, beating Pakistan in the final. Gambhir performed well in the shortest form of the game, ending the tournament as India's top run scorer, with 227 at an average of 37.83, including three half-centuries which included a crucial 75 runs off 54 balls against Pakistan in the final.[5]

2008 started well for Gambhir. At home, he scored an unbeaten 130 in the Ranji Trophy final to help Delhi beat Uttar Pradesh by nine wickets just two days before the team for the ODI tournament in Australia was to be announced.

Gambhir was forced to miss the Test series in Australia due to a shoulder injury. In the 2007-08 CB Series, he scored an unbeaten 102 at The Gabba against Sri Lanka in a match washed out due to rain. Three weeks later at Sydney, he scored a career-best 113 off 119 balls against Australia, in a high scoring match which India lost by 18 runs. He finished the CB series as the leading run-scorer with 440 runs.

In 2008 Gambhir finally solidified his place in the Indian Test team with a string of high scores. Opening the batting with Delhi teammate Virender Sehwag, he scored 858 runs at over 61 in seven matches as of December including a double century against Australia in the Border-Gavaskar Trophy. However in the same match he was involved in controversy when he was found to have thrown an elbow at Australia all-rounder Shane Watson while taking a run. Gambhir denied the charges in a media conference, claiming the elbow was unintentional, but pleaded guilty in the hearing and served a one-Test ban for the incident. Despite missing the last match, Gambhir still topped the run-scorers list for both teams.

He was the leading run-scorer in the Test series against England in December 2008 and against New Zealand in early-2009, meaning that he had achieved this feat in three consecutive series.

Gambhir played his first major Test series outside the sub-continent, having toured New Zealand in 2009. In the second Test match he scored a match saving 137 in the second innings. He stood more than five sessions in the middle and faced over 430 balls. This innings led Virender Sehwag, Gambhir's opening partner, close friend and captain for the match, to call him 'The Second Wall' in reference to Rahul Dravid. Gambhir, with 445 runs in six innings at an average of 89, helped India win 1-0 to script a series win in that country after 41 years.

He was named as the ICC Test Player of the Year for 2009.