I like Gaia but as of now I don't think the developer is focused on fixing the issues and performance problems it has. It's all cpu driven and doesn't support multi-tiles. He had a very cool beta back in the day but dropped it in favor of a new thing he was working on at least according to my knowledge. So probably won't be Gaia that'll get acquired.
However I too believe Terrafirma is the next big thing for terrains. Personally though I prefer a voxel based solution that lets me make caves. But I also have a feeling Unity is repurposing polybrush for the same purpose.
There are some upcoming contenders as well. More tools are popping up for terrains. You guys probably know @antoripa 's assets by now. Sentieri the path creation tool and cascade the river tool . Both are great assets. 2018 is the year of terrains