Per the document here:  Dynamics 365 service description - Power Platform | Microsoft Learn. hyperlink “Product and Feature Availability”

shows the current status of the following feature as "not available"

Embed a model-driven app as a tab app in Teams

This is a significant gap in Teams/Power Apps integration functionality. Both GCC and Commerical users can use Teams Web to embed and connect to Model Driven Power Apps on a channel tab, but the Teams Desktop experience for GCC users is broken.

Please fix this ASAP.