To take your question literally: average elevation gain. Since most of my rides begin and end at my garage or to somewhere, then later back, almost exactly zero.

Most of my rides (by number) are in town and back. I record that as 550'. Recent months, I have been doing little in hills so I record my rides through the Tualitin valley as flat though little of it is actually. If I ride up to Skyline over Portland, getting to 1000' is easy. Bald Peak southwest of Portland is about 1600' so that's usually a 2000' day.

I do not have a computer with elevation function on any of my bikes. I count the elevation gained on specific hills, not gradual inclines or small stuff.

Ben