Elevation gain values from your treadmill can be entered into your workout - it has to be entered online through Garmin Connect website, click "Add more data" at the bottom when filling out the manual activity.
Slope % however can not be entered... to track hill climbs, of course you could calculate gain (rise) if you had average slope % and distance traveled. Other options i find helpful while on a treadmill is just to track what a typical hill climb for your might be... so for me I set it at 4% for 1/4mile which is a soild 50' hill. If you did that a few times over the course of a treadmill run.... it would be easy to remember - "3 hills"...x50' = 150' gain for the run.
The mobile App (at least droid) doesn't have that option to add HR data, Elevation, etc.
