Authored by @RichardWakeman
You can in fact have a GAL that spans multiple tenants, to include a cross-sovereign cloud solution with a tenant in the Commercial cloud along with GCC High. This is to help those organizations that must deploy into both a Commercial tenant and a GCC High tenant at the same time. Many of those organizations have a requirement to light up a consolidated address book across both tenants.
Read more about this on the Public Sector Community Blog.