While the official Gamebryo engine is a commercial product typically licensed by professional studios like
For historical or research purposes, contact Gamebase for licensing terms. The engine is no longer actively promoted for new projects.
The long answer: Do not waste your time. The "full engine" is a relic. Modern modding tools (NifSkope, GECK, xEdit) are superior to the original SDK in every way. The best code is the code that works, and the Gamebryo engine "full download" from 2005 will likely crash on your Windows 11 machine.
Conclusion