You may need a splitter that will divide the Plantronics gaming headset's plug into two circuits: one circuit being for the headset audio and the other circuit being for the headset microphone.
The adapter will have two cables that, in turn, go to the respective headset and microphone ports (3.5mm) on your desktop.
Start by googling "Tip Ring Sleeve". Determine what you have with respect to 1) the desktop and, 2 the gaming headset.
For both headset and microphone you must first have the necessary electrical connectivity and then the necessary sound card configuration with respect to Playback devices and Recording devices.
Right click the small speaker icon (usually lower right screen corner) to delve into the available settings and...