here is a preset .tgc for importing correctly Gaea heightmap into terragen.
right click on terragen node editor -> insert clip file and pick the file, it will create 4 nodes, load the gaea heightmap on the GaeaLoad node, and connect the heightfield shader to the compute terrain node.
rename the gaea.txt to gaea.tgc because forum didnt allow me to upload the .tgc file for some reason