The Mixed Reality Plugin has been removed in 5.1, and is not required to use G2. However, it’s recommended to enable the “HP Motion Controller”-plugin, as it will give you the HP Reverb Inputs.
Make sure that you have the Windows Mixed Reality runtime installed and set up (Windows Mixed Reality Home should be working prior to running the Editor). You should also make sure that Windows Mixed Reality is set to be the active OpenXR runtime, this tool is great to check which OpenXR runtime is currently active, and to switch between them: GitHub - maluoi/openxr-explorer: A cross-platform OpenXR capabilities explorer and runtime switcher with a CLI and GUI.