This may not be the "cleanest" way of doing it but if you take the miles traveled in feet, multiply that by the incline, and divide that by the hours it took you and then multiply that by the speed you're traveling, I think that will give you the answer you are looking for.
If I walked 1 mile (5280 feet) at a 15% incline for one hour at 3 mph I will have been climbing at the rate of 2,376 feet per hour
5280 x .15 = 792 feet / 1 = 792 792 x 3 = 2,376 feet of elevation per hour
I think this is correct because it closely matches what the treadmill I use says in my workout summary when I do 45 minutes at 3.5-3.8 for 45 minutes plus cooldown...
Any mathematicians out there?