Once you start X-plane in the Joystick/Equipment menu you must select your com port (Garmin section) and check the box (see image below).
From the tools menu in the GPS, you MUST change the serial data interface mode to “Aviation In.” Without this setting, the GPS will not connect. This menu will be found in the Tools/Settings/Interface menu on your GPS depending the model.
EVERY time you start the GPS for use with a simulator you must ALWAYS place the GPS in simulator mode for the PC to communicate with the handheld unit. Garmin does not allow this setting to be saved, so unfortunately you MUST, set the handheld in sim mode if the hand held is turned off. Sim mode disables any form of weather input if your device supports it.
This is how you set the 796 in “simulator mode”
This is how you set the 696 in “simulator mode”