Tried to use the usual GasPlanet texture without heightmap, but that won't work. Is there a way to cut out continents shape from the texture but still place production tiles on them? Probably forming a *.dds file that uses the whole area as continent but with transparent texture might be an idea.
I was able to make a brand new custom continent using my own textures and the way i believe is works, as you said, is it uses the texture in order to determine the planets Max PQ and other things.
What i might suggest doing is create a height map identical to the base map, however change the alpha channel to 99.9% black meaning it would be practically invisible. However also note, i believe you cant just have an entire map as a continent there needs to be some pure black areas for the map to be recognized correctly.
If that is the case i would also suggest, assuming your having a traditional "stripy" planet, is to black out everything on the height map except for a few specific streaks. That way when the game loads the continent that then apply over the top of the base map, it wont be noticeable as the color change will be in areas that merge into the map seamlessly. This is what i ended up doing for the custom planet im working on. Likewise if you want you could add an image of something cool, like im doing, such as a space station or similar image that represent the habitable area that way it wont look out of place. however depending on how much PQ you want to base planet to have that could come up problematic and look to busy.
If your using Photoshop i can give you the correct .dds file settings i used in order for it to correctly work.
Also if you dont 100% get what im suggesting i can work you though it more, let me know! 
Edit: Also dont forget to change <Alpha> for <BaseLayerDiffuseColor> to 255 (default 150) so its not transparent, that actually alters the base colors which means they wont match. it should look like this
<BaseLayerDiffuseColor>
<Red>255</Red>
<Green>255</Green>
<Blue>255</Blue>
<Alpha>255</Alpha>
</BaseLayerDiffuseColor>
