The cricket board had ordered an investigation last month following a complaint from team manager Asanka Gurusinghe against the 26-year-old player.
Sri Lanka Cricket (SLC) said Gunathilaka was also fined 20 percent of his annual contract fee for violating the board's constitution as well as the terms of his contract.
"The charges relate to misconduct and behaving in a manner unbecoming of a player, and the breach of several clauses of his contract which relate to fitness and discipline," an SLC official told AFP.
An SLC statement said Gunathilaka pleaded guilty to the charges without spelling out what exactly they were.
Sri Lanka suffered the embarrassment of losing all nine international matches in their home campaign against India.
Virat Kohli's visitors took the Test series 3-0 and then won a one-day international series 5-0 before triumphing in the lone Twenty20 international by seven wickets in Colombo earlier last month.
Gunathilaka has also been dropped from the squad for a five-match ODI series against Pakistan in the UAE.
Gunathilaka, who made his international debut in 2015, averages 42.41 in 12 ODI matches he played this year.
First Published: October 5, 2017, 3:09 PM IST