After a season back in English football with Swansea City, he joined Tottenham Hotspur for a reported £8 million transfer fee.

[7] Two weeks later, he made his debut in the next round against Luton Town, entering the match as a 59th-minute substitute for James Harper in a 5–1 win at the Madejski Stadium.

He returned to his parent club and featured in Reading's 2–0 loss to Cardiff City in the FA Cup third round on 3 January 2009.

[15] He scored two further goals for Crewe, against Milton Keynes Dons and Cheltenham Town respectively,[16][17] but could not prevent the club's relegation to League Two.

[18] Gylfi scored his first Reading goal against Burton Albion from 35 yards out in a League Cup first round tie at home on 11 August 2009.

[20] Gylfi was a major contributor in Reading's extraordinary 2009–10 FA Cup run which included wins over Premier League sides Liverpool, Burnley and West Bromwich Albion.

[25] Before the match, Gylfi was named as the 2009–10 Reading Player of the Season, surpassing Jimmy Kébé and Ryan Bertrand, who finished second and third respectively.

[27] His performances and his young age prompted several Premier League clubs to enquire over his availability, but he committed his future to Reading when he signed a three-year contract in May 2010.

The evening after the match, the Icelandic media reported Gylfi was to travel to Germany the following Monday to undergo a medical at TSG Hoffenheim ahead of a transfer.

[37] On 1 January 2012, it was announced Gylfi would join Premier League club Swansea City on loan for the rest of the season.

[43] As a result of his fine form, Gylfi was named the Premier League Player of the Month for March, the first Icelander to win the award.

[56] In a home match against Arsenal on 9 November 2014, Gylfi curled a free-kick over the wall and into the corner of the net from 25 yards to equalise in an eventual 2–1 win.

[58] On 17 January 2015, a wayward pass by Gylfi allowed Chelsea's Oscar to score after 50 seconds in an eventual 5–0 home defeat for Swansea.

[59] A week later, he scored and was sent off in added time at the end of a 3–1 defeat away to Blackburn Rovers in an FA Cup fourth round tie, with Swansea already having been down to ten men after the earlier dismissal of Kyle Bartley.

[63] In summer 2017, Gylfi refused to participate in Swansea's pre-season tour of the United States, proclaiming he was not in right frame of mind to go with the club because of uncertainty over his future.

[65][66] Eight days later, he scored his first goal for Everton while making his full debut, with a shot from 50 yards in the UEFA Europa League play-off round second leg against Hajduk Split.

[73] Gylfi scored his 100th goal in English football during Everton's second round EFL Cup tie against Salford City on 16 September 2020.

[78] He made his debut on 22 September in his first professional game for over two years, as a 70th-minute substitute for compatriot Sævar Atli Magnússon in a 1–1 home draw with Vejle Boldklub.

[80] On 14 March 2024, Gylfi signed a two-year contract with Valur, thus joining a Besta deild karla team for the first time in his senior career.

[83] Gylfi took part in the 2008 U19 European Championship qualifying, scoring twice in qualification and twice in the elite round, but Iceland lost out to group winners Bulgaria by three points and missed out on the finals.

[84] He made his first appearance in the 2009 European Under-21 Championship qualifying four days later, coming as an extra-time substitute, and played in the remaining matches.

[86] In May 2010, Gylfi made his full Iceland debut in a convincing 4–0 win over Andorra and provided the assist for the second goal from a free-kick.

[89] On 13 October 2014, Gylfi scored both goals (one of which was a penalty) as Iceland defeated the Netherlands in a UEFA Euro 2016 qualifying match.

[90] In the reverse fixture the following 3 September at the Amsterdam Arena, he converted another spot-kick for the only goal of the match, after Gregory van der Wiel fouled Birkir Bjarnason.

On 18 June, he scored Iceland's goal in their second group stage match against Hungary, a 1–1 draw at the Stade Vélodrome.

[101] On 16 July 2021, the then 31-year-old Premier League footballer was arrested "on suspicion of child sex offences" and was released on bail while investigations continued, stated Greater Manchester Police (GMP) without naming the player.

[102] Icelandic newspaper Morgunblaðið reported that the player arrested for alleged child sex offences was Gylfi.

[106] In October 2022, with Gylfi not charged but still under a travel ban, politician Njáll Trausti Friðbertsson said that he would discuss the player's case with the Foreign Affairs Committee.