Download the General Random Package via the Plugin Manager in Unity by selecting Packages: My Assets in the Package Manager. If you don’t have the Package Manager tool window, you can enable it via the Menu Window -> Package Manager.
Select the General Random Package click on Download on the right side window pane.
After a short download you can Import the package into the project.
Select All to import all available files, including tests
Tests are not required for operation and can be skipped. These can be useful to test targets or to see how the package works.
Continue with Getting Started or use the GRandom struct in your code, via new GRandom(<unique seed>).