What department of the US military is USGI? What's its full name? I really appreciate it． . .

First of all, USGI has more than one meaning, and one meaning is the global investor of the United States, which has nothing to do with the military.

The full name of another USGI is: Universal strategic groups Company, which is one of the companies that distribute materials to the US military. It is responsible for the sales of various materials, including weapons, by the US government and military. Common so-called USGI weapons (and other military products) are sold by the US government through this company.