GameInfo\GalaxyScenarioDef.galaxyScenarioDef
Needs to be added to OrbitBody list and update count.
PlanetType and update count.
Then the lists near the top for map making, update counts.
Do a search in the file for a planet type like ice and see how it is added.
What I did was download Sins Plus and use notepad++ compare feature to compare the 2 GalaxyScenarioDef.galaxyScenarioDef files and see all that they added.
If you want to use galaxy forge to make maps with the new planets, copy the GalaxyScenarioDef.galaxyScenarioDef into the GF directory replacing the one included after you backed up the original somewhere.
Did you add all the icons to Textures\Icons_planets and all the new brushes to the Windows folder?