HighSide for Teams supports all areas where 3rd party apps are allowed within Microsoft Teams. Currently, 3rd party apps are not allowed within GCC High nor in DOD Environments however they can be enabled in standard GCC environments. This link will take you to Microsoft's documentation around 3rd party apps in GCC, GCCH, and DOD environments which will give some additional information for what is to follow here. In order to enable 3rd party apps within GCC environments simply have your admin navigate to admin.teams.microsoft.com, Click "Manage Apps" and click "Permission Policies", Click the Global (Org Wide Default) option, under the "Third Party Apps" section set this to "Allow All Apps" (Or you can simply set it to allow specific apps and block all others and select "HighSide" as the app allowed). Once you've edited this setting you'll be able to setup HighSide for Teams in the same way as other environments setup HighSide for Teams (For example you can follow this guide to setup HighSide for Teams in a channel)