I would think that it is possible though. Worst case scenario: just lean over the edge and do your thing. Or let somebody else do the scouting or Flame Throwing.

I agree that it's been underutilized. On the other hand: the readers have had years of thinking to do this, the characters in the story have only been using Gateways for, what? One year? Six months? And it's been rather busy in the meantime.

Have we ever even seen if it is possible to have a gateway that is not vertical? Or that it can be made at any other point than with the bottom on solid ground?

There have been a large number of philosophical debates on using Gateways. I.e. opening the exit of a horizontal Gateway on top of the entrance would make somebody fall indefinately. You can open a Gateway to the Sea of Storms and flood the army you're opposing. (It would look hilarious if a Trolloc is being chased by a snapping shark.)