What is the gain/loss on the drive summaries expressed as ft (or meters)? Is that elevation?

GPS co-ords as best I can tell (unknown what they use to turn those into altitude as many folks have driveways that differ from the nearest road to say nothing of multi-story parking structures). It certainly isn't barometer based. 

is it based on barometer or google maps ?

This shows the difference (vs. sea level) between your starting and ending locations. It is somewhat lossy due to sampling errors (i.e., a round trip drive may show an overall gain/loss). Supposedly this is because (for example), it might sample your trip start height after the car is in motion and thus miss the couple of feet of change as you deal with driveways.

I notice this often because my driveway and street have a 6' or so drop (plus I do a 3 point turn into a much steeper driveway when I visit family so on those trips I take a hit at that end too).