The Seoul property was closed on 24 November, with the Busan venue temporarily shut down on 1 December.
The new closures are in line with COVID-19 safety measures, as country-wide restrictions were extended for a further two weeks.
The Seoul Capital Area is currently under Level 2.5 alert, the highest of the five alert levels in South Korea, and the rest of the country is under Level 2.
The affected venues are Gangnam COEX and Gangbuk Millennium Seoul Hilton in Seoul, and Seven Luck Casino in Busan Lotte.
GKL has subsequently raised projected losses from KRW27.05bn ($24.5m) to KRW32.58bn.
This loss has been gradually rising with each extension, from the original amount of KRW5.1bn. The sums are based on average net sales from July to September 2020.
GKL is a subsidiary of the Korea Tourism Organization and is indirectly affiliated with the country’s Ministry of Culture, Sports and Tourism.
Another casino operator in Seoul, Paradise Co, had previously closed its doors on 15 December but resumed operations on 4 January.
Guests to the venue are required to show negative test results to be allowed in and there are additional COVID-19 countermeasures in place.
