Date of birth
Gunnarsson was brought up in his native Iceland, progressing through the Breidablik Academy. By the age of 15 he was already playing Under-19s football in the UEFA Youth League.
A loan spell with Reykjavik Athletic (IR) gave him a taste of second division football, before the summer of 2018.
That year, Patrik joined Brentford B on a two-year contract including a further year option. Injuries to goalkeepers Luke Daniels and Ellery Balcombe saw him jump up the queue to make the bench towards the end of his first season in West London.
His Bees debut came on 9 March 2019, coming on as a 74th-minute substitute for the injured Daniel Bentley in a 2-1 victory over Middlesbrough – the first such win for Brentford in 81 years.
During that season, Gunnarsson was also part of the B Team squad that won the Middlesex FA Senior Cup, keeping a clean sheet and defeating Harrow Borough 4-0 in the Final at Griffin Park.
In summer 2019, Patrik signed a new four-year contract with optional year before he was promoted to the First Team squad in September. On 20 February 2020, Patrik joined League One club Southend United on a seven-day emergency loan, which was subsequently extended by two weeks. He returned to Brentford on 15 March, after making three appearances.
On 7 September 2020, Patrik joined Danish First Division club Viborg FF on loan until 31 December 2020. Patrik made 12 appearances during his spell and on 5 January 2021, he returned to the same division to join Silkeborg IF on loan until the end of the 2020–21 season. Both clubs' promotions were confirmed with four matches to play and Patrik finished his Silkeborg spell with 14 appearances.
In March 2021, Patrik was named in Iceland's 2021 UEFA European U21 Championship finals squad.