You can reveal Watchtower to trash a gained Nomad Camp. This is because after Nomad Camp was gained, it's exactly where Watchtower expects it to be, which is on top of your deck. Watchtower isn't automatically looking in the discard pile for gained cards, it's looking for wherever those cards were gained to.
With Transmogrify, it should work exactly the same. Transmogrify moves the card from where it expects it to be (on top of your deck) to your hand.