Hi,
Is it a character? It should conform to the terrain.
else, you need to raycast down from the gameObject, and get the hit position, that's the position you would set the gameobject. Warning this will have a side effect of having the gameobject half under the terrain, so you likely want a dummy gameobject representing the bottom of the gameObject and attach the gameobject to that. and have that dummy follow the terrain. Or you translate the hit position accordingly.
bye,
Jean