Should steaming water be used for stock/gravy?
It shouldn't matter if you re-use the water you used for steaming or use fresh water. I would only use it in a few situations:


You want to season the gravy with whatever flavors ended up in the water
You want some of the starch in the water to help thicken the gravy
You want to be frugal and reuse the water and not pour it down the sink


The benefits of one and two are mostly negligible because the amount of flavor or starch isn't going to be that much.

Situation three is really the only reason I would reuse the water, but most of the time cooking water ends up watering the plants.