He joined Cardiff City's youth system and was signed by Premier League club Tottenham Hotspur for £4 million in January 2008.

[5] He signed his first professional contract with the club on 1 August 2006 along with fellow Cardiff academy graduate Darcy Blake.

[6] Primarily a right-sided defender, Gunter made his senior debut for Cardiff on 22 August 2006 against Barnet in the League Cup first round, playing the full 90 minutes of a 2–0 loss at Ninian Park.

[7] After being included in the first team squad on several occasions,[8] his league debut came in a 1–0 home loss to Queens Park Rangers on 17 November 2006.

[14] At the end of the 2006–07 season, having made sixteen appearances in all competitions,[15] he became Cardiff's youngest player to win a senior international cap after making his debut, beating the 48 year old club record held by Graham Moore.

[16] According to reports on 21 December 2007, Cardiff agreed a substantial fee with Tottenham Hotspur and therefore allowed Gunter to enter into talks with the club.

[18] Gunter made his debut for Spurs on 15 January in an FA Cup third round tie against Reading at the Madejski Stadium, playing the entirety of a 1–0 victory.

[19] Fifteen days later, he made his top-flight debut for Spurs, playing 62 minutes of a goalless draw at Everton before being replaced by Kevin-Prince Boateng.

[20] He made only one more league appearance that season, filling in for Didier Zokora for the final 17 minutes of a 4–1 loss at Birmingham City on 1 March.

In the match between Nottingham Forest and Crystal Palace on 23 March 2010, Gunter made his 100th senior club appearance.

[30] Gunter completed a move to Reading on 17 July 2012, signing a three-year deal for the club who had been promoted to the Premier League.

[33] He scored his first goal for Reading against Peterborough United in the League Cup on 28 August 2012, the 3–2 home victory putting the team into the third round.

[36] In the League Cup first round on 11 August, he scored the extra-time only goal as Reading won away at Colchester United.

[53] Gunter missed two games in March 2023 due to his new commitment to coaching the Wales national team; the decision was endorsed by Jackson.

[57] Gunter received his first call up to the senior international side at the age of seventeen for a friendly match against New Zealand at the end of his first season of professional football, describing himself as being "absolutely staggered".

On 8 October 2010, Gunter received a straight red card in added time for a professional foul against Dimitar Rangelov in the 0–1 home loss against Bulgaria in a Euro 2012 qualifying match at the Cardiff City Stadium.

[63] He captained the side for the first time for Wales' friendly with Panama on 13 November 2017, in which he equalled his former manager Gary Speed's record of 85 international caps for an outfield player.

[65] On 20 November 2018, he surpassed Neville Southall's record of 92 caps for Wales in a 1–0 friendly loss away to Albania, in which he was captain for the second time.

[69] In November 2022, Gunter was named in the Wales squad for the 2022 FIFA World Cup in Qatar,[70] but did not play any of their three Group B games.