If I have understood correctly, Providing the collisions are set up correctly (assigning EntityTypes to the player entity and the terrain, you can have gravity acting on the player constantly and it wont 'fall through' the terrain.
However EntityY#(player_entity_name) and TerrainY#(terrain_name) are the commands you're looking for.
However EntityY#(player_entity_name) and TerrainY#(terrain_name) are the commands you're looking for.