O365 GCC High is a copy of the DOD cloud environment for use by DOD contractors and cabinet-level agencies as well as cleared personnel. One critical distinction: when handling classified data, environments have a high side and a low side, the high side existing so users can handle classified data. GCC High is NOT a high side environment. It received its name because it meets FedRAMP high impact requirements.
For many government standards, one must make sure anyone working in the environment meets the requirements of specific government background checks. GCC High acts as a data enclave of Office Commercial. It's compliant with DFARS, ITAR, NIST-800 171, and NIST-800 53.
Regarding feature parity: Microsoft does not offer any calling plans available in GCC High. There's also often a 10-13 month gap between when features are available in Commercial and when they become available in GCC High. Know more about Office 365 GCC High Pricing in detail.