I believe for that feature, you'd have to do it one at a time...when I tried to bulk edit a few of my saved listings as a test, this is all I got:
Make Editable:
All
Title
Start Price
BIN
Reserve
Quantity
Duration
Nothing about gallery..maybe someone else knows how to get around this? It might be possible to do it thru ebay once it's live, but I'm not sure.
Had another thought tho--go to your profile tab, and checkmark gallery, then save it. That will at least save those preferences for future listings.