Using Microsoft Power Automate flows, we were able to implement the majority of the Slack / Azure DevOps integration functionalities (build notifications, code push, and so on). Check to determine if you have Power Automate if you have a GCC Office365 subscription.
Power Automate has interfaces for MS Teams and Azure DevOps that provide triggers and actions that can produce nearly the same types of messages as Slack. While Adaptive Cards aren't yet supported in GCC, you can use HTML and dynamic content to structure the message.
The Azure DevOps tools for Teams don't work in GCC (yet), however you might be able to get over some of these limits with Power Automate.