As you said, Organization assets library is not available in GCC high tenant.
As a workaround, we could create a custom content type to set up PowerPoint template in Content Type Hub, then publish it across all site collections.
Here are steps:
- Access to the content type hub site which SharePoint Online already provided:
https://YourTenant.sharepoint.com/sites/contenttypehub
2. Create a custom content type based on Document, then upload PowerPoint template to it:
3.Then click on "Manage publishing for this content type" option in that content type page to publish this content type:
4.Now go to the SharePoint Online Site Collection Administration and select your Site Collection where you want to use the new content types (Site Settings -> Site Collection Administration -> Content Type Publishing).
5.In the “Content Type Publishing Hubs“, Enable the checkbox of “Refresh all published content types on next update” and then click on “OK“, this will take a while to take effect, per my test it has been successfully loaded under Subscribed Content Type:
6.Now in the below screenshot, You can see my custom Document content type in the list of Site Content Types in the site where I want to use it.
7.Lastly, we could add this content type in document library to use the published template in all site collections.
More information for your reference:
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