You are using the Mendix Web Modeller? Did you tried to see the Widget in the Desktop Modeler?
in the modeler, click on AppStore button at the top right:
There search for the widget you want to get added to your app and click on the download button:
This will make the widget get downloaded and available in your app.
To see it in your app go to the project directory, /widgets and you will see GeoLocationForPhoneGap.
To use it, go to any page or snippet, rightclick on the canvas, “Add widget” and search for geolocation: