In Gmax you would just use a “mirror” modifier applied to each mesh and check the button for whichever axis you need to mirror. If the origin for each mesh is not set to 0/0/0, you’ll need to use the inverse of the position for whichever axis you mirrored (e.g., if mirroring the X axis and the mesh’s origin is at +100 X, you’ll want to move it to -100 X).
Veltools can mirror a whole tile, but no matter if you do it mesh by mesh or all at once, there’s one thing you shouldn’t forget: The textures of cliffs or walls und the ground textures including water have to be remapped as well or you will get seams.