Gunathilaka made his cricketing debut in the 2007–08 Inter-Provincial Twenty20 tournament for Sri Lanka Schools, alongside fellow Colts Cricket Club player Kusal Perera.

Gunathilaka made his List A debut for Colombo Cricket Club against Sinhalese Sports Club in 2009–10, scoring a single run.

In March 2018, he was named in Kandy's squad for the 2017–18 Super Four Provincial Tournament. The following month, he was also named in Kandy's squad for the 2018 Super Provincial One Day Tournament.

In August 2018, he was named in Dambulla's squad the 2018 SLC T20 League.

He made his One Day International debut for Sri Lanka against the West Indies on 1 November 2015.

After Kusal Perera dropped from the 2015–16 New Zealand tour due to drug scandal, Gunathilaka was included to the ODI squad for opening position with Tillakaratne Dilshan. His batting ability did not show in the first two ODIs, but in the third ODI, which was his only 4th international match, Gunathilaka scored a fast and dashing 65 runs from 45 balls. The innings included 7 fours and 4 sixes and getting the match away from New Zealand. Sri Lanka won the match by 8 wickets and Gunathilaka adjudged man of the match for his performance.

During the same tour, he made his Twenty20 International debut for Sri Lanka against New Zealand on 7 January 2016.

Gunathilaka was not initially announced to the Sri Lanka squad for 2017 ICC Champions Trophy. But he was called for the squad for injured Chamara Kapugedera. He scored 77 runs against India, guiding the team to victory. Sri Lanka chased 321 at the end, recording the highest chase ever in Champions Trophy history.

Gunathilaka scored his maiden ODI century on 6 July 2017 against Zimbabwe at MRIC Stadium. He along with Niroshan Dickwella added 229-run partnership for the first wicket, where Dickwella also scored his maiden ODI century. Sri Lanka chased 310 to win the match, which is recorded as the first 300+ chase by Sri Lanka in home soil as well. Gunathilaka won man of the match award for his impressive batting performance.

In the fourth ODI against Zimbabwe, he and Niroshan Dickwella set the record for becoming the first pair of batsmen to score two successive 200-plus partnerships in ODIs. He missed out his consecutive century, dismissed for 87 runs, whereas his counterpart Dickwella scored his second consecutive century.^[13]

He made his Test debut for Sri Lanka against India on 26 July 2017.^[14] He only scored 18 runs in the match, and took the sole wicket of Abhinav Mukund. Sri Lanka finally lost the match by huge 304 runs.

In May 2018, he was one of 33 cricketers to be awarded a national contract by Sri Lanka Cricket (SLC) ahead of the 2018–19 season.^[15][16] However, in July 2018, SLC suspended him from all forms of international cricket, after he breached the code of conduct. Despite his disciplinary issues, he helped Sri Lanka to have a firm opening stand in the second test aga