Terrain Mesh into Terrain Entity
GenSurf outputs its terrain as a func_group entity. If you create a terrain mesh by any other means, you need to group it as a func_group entity.
Make certain that all the non-visible surfaces of the func_group brushes are textured with common/caulk.
Texture the visible surfaces with common/terrain or common/terrain2.
Select the func_group and turn it to detail content (CTRL M) or use the Selection menu.
It is possible to map terrain onto something that is not a “terrain” mesh. Keep in mind that textures will be planar projected onto all surfaces in the terrain entity and that brush vertexes will be used to determine where blends start and end.