try to triangualte meshes in DCC (blender for example) first, before importing.
if you use FBX, set normals/smoothing to "Faces" in blender FBX-export settings.
Set "Z" to be the Up-Axis and I think it was -X for "forward"
for heightmap-export from Gaea use:
2017x2017 Grayscale, 16-bit precision (PNG) - you need to use "resample" option from the build settings!
If you set the height to be max at 4096m in Gaea, the math/settings for import in UE will look like this:
2017x2017 with 2x2 components. Editor scale (100, 100, 800)
Altitude range 0-4096 meters in Gaea = Default range in UE4 is 512 meters at landscape Z-scale 100.
A landscape Z-scale at 800 gives 8x512 m = 4096 m. (so it should be scaled right)
hope that helps!