Since Jira Service Desk 4.12, support to connect mail channels to Office 365 accounts using OAuth 2.0 has been introduced. However, it doesn't support GCC (Government Community Cloud) accounts.
It isn't supported because regular Office 365 accounts connect to outlook.office365.com whilst GCC accounts connect to outlook.office365.us.
Therefore, the connection is not established and the mail channel can't be connected using OAuth 2.0
Manually update the hosts file of the operating system to point to the US domain. However, doing this will remove the automatic failover to a new IP address in case of outages in the primary IP address.