GlobalProps is a class that holds...global props
https://files.inductiveautomation.com/sdk/javadoc/ignition81/8.1.27/com/inductiveautomation/ignition/common/project/GlobalProps.html
You obtain an instance of the class by deserializing the project resource, e.g. with ResourceUtil:
https://files.inductiveautomation.com/sdk/javadoc/ignition81/8.1.27/com/inductiveautomation/ignition/common/util/ResourceUtil.html
You obtain a ProjectResource from the project system; how you do that depends on what scope you're calling this from. Ignition Extensions can help with that.