Today with 1.21 we finally get a track editor, along with a GT6 track Path Editor App on iOS and Android tablets. I've toyed around with it and it is very powerful as far as designing a track is concerned - if you choose a flat track area you can even use a reference image - but unfortunately you are not in control of the terrain for raising or lowering the track, nor can roads cross - no bridges or tunnels.
It took me about 5 mins to create a track that matches an image of an existing track (Zandvoort), though I didn't get the scale right immediately.