These creatures live inside dungeons.. They look like open doorways.
They are inevitably found in pairs. The peculiarity of the gates is that anyone who attempts to walk through the 'doorway' will be repelled by an invisible force, unless someone walks through the gate's partner at the same time.
The effect is that groups of adventurers must split up, and that lone adventurer may not proceed. However some gates may be 'tricked' by throwing an object through one gate at the same time as the group proceeds through the other.
Some AND gates only bar passage one way, generally the way that leads further into the dungeon, so that a group must split up to proceed but one sub-group need not wait on the other to return to the surface.