Gimp and Height maps do not mix well
The gimp is only 8 bit
( gimp 3.0 will handle 16 and 32 bit images)

That means there is ONLY 256 tones ( 0,255)
that will give you a DEM / normalmap that is STEPPED
you NEED to use at least a 16 bit image !!!

A 16 bit image has 65536 tones
Ushort / Unsigned = 0 to 65535
Short / Signed = -32767 to 32766
or a 32 bit floating-point image ( normally 0 to 1 with 32 decimal places )


i use different software to make a 32 bit floating point or 16 bit Unsigned "Noise" image "FBM" is one i like also segmentation of triangles works to get the original height map
then use some custom software to convert that to a NormalMap that will render Correctly on a sphere


Now what I want to do is to set a height level, which I believe for land needs to be 0 (low) to 1 (high), with an appropriate number for each layer, and then normalize or smooth or whatever across all of this. Then I can apply a gradient color green, and then do it again only at the higher levels and do it more brown, and so forth.
for that i would use "G'Mic" a terminal based image lib
-- you WILL need to build the current source code to reenable 16 bit output
G'MIC - GREYC's Magic for Image Computing: An Open and Full-Featured Framework for Image Processing


it sounds ( " 0 (low) to 1 (high) " ) like you have a 32 bit height map
for use in gimp
renormalize the different color layers ( deep sea to land ) .Normalize them to 0 to 255
and save as a 8 bit char image

then use gimp to colorize them

Gimp has a handy built in "colorize" tool that dose a great job

Gimp > "colors / map / sample colorize"
just have the black and white and the color one open