Hey, so I’m not 100% sure what you mean with linking the grid manager to the landscape. The grid manager has options for automatically generating a walkable grid on top of any underlying terrain (see the first four tutorials of my tutorial playlist which goes into this generation).
But if you want to get into full procedural generation of a Civ-style map with intelligent placement of coastlines, mountains, rivers etc. that is a huge topic, and ATBTT does not include something like that by default. The underlying structure of ATBTT should be friendly for adding that sort of thing, though. I recommend looking at Red Blob Games’ excellent tutorials on procedural map generation on grids: https://www.redblobgames.com/