I use the NPE Runn, it's a fantastic device (particularly if you own a treadmill, not using a public/gym treadmill).
You can add the NPE datafield to your treamill activity and it'll record the incline percentage as a chart, and also record the value of your overall elevation gain for the activity. Do note that since it's not a native Garmin product, those two pieces of information won't factor into anything in the Garmin ecosystem at all, they are simply displayed in your activity for your reference later.
