Most of the tree creators output b3d models, so those just import normally into level/terrain editing tools. Obviously TreeD is designed for use with gile[s], although those trees also just import like any other object if you convert them to b3d. The other tree generating tool I know of is Sabata's automatic foliage system; don't know anything about that.
Most of the terrain editing tools I've seen seem to use Blitz terrains, although the best one IMO is ALE and that outputs b3d meshes. Thus it is about the only one where you can lightmap the created terrain in gile[s].
I recommend using ALE to create your terrain, then take it into gile[s] to place trees and lightmap. That's assuming you need the level of control/detail that ALE offers; I don't, which is why I haven't purchased it (yet.) You can get some pretty detailed looking terrain using a base texture for color and a tiled, black/white, multiply blended detail texture.
Most of the terrain editing tools I've seen seem to use Blitz terrains, although the best one IMO is ALE and that outputs b3d meshes. Thus it is about the only one where you can lightmap the created terrain in gile[s].
I recommend using ALE to create your terrain, then take it into gile[s] to place trees and lightmap. That's assuming you need the level of control/detail that ALE offers; I don't, which is why I haven't purchased it (yet.) You can get some pretty detailed looking terrain using a base texture for color and a tiled, black/white, multiply blended detail texture.