Heavy Gravity Worlds were in GC II. They might have been in at one time as part of the devlopment cycle for GC III, but replaced in favor of Frozen Worlds.
The problem with all these theories is the icons still exist in the game folder along with Unstable world (Lava) texture and gradiant So it appears that they may be in, soon, or in DLC's expansions, maybe they are holding them off till release
Well, I asked a while back in one of the devstreams about more Extreme Worlds coming in, and they said that they weren't thinking about doing it at the moment because it took too many normal habitable planets away if you have too many different type of Extreme Worlds (though more could always be modded in). My reckoning is they just never removed the icons, like they never removed the "Zombie Hipster Flesh" description.
Still, I can see more coming in via DLC/Expansion. Say if they limit the overall number of Extreme Worlds to X and then subdivde all of those up between A, B, C, D, E, F, G, and H.