As far as I know it isn't so easy a copy and paste, if you want to adjust the gravity by yourself or random you need to change the code, at the moment the gravity is fixed to the planet as much I understand
I haven't looked at the code, but I don't see why it would be so difficult. I don't remember if GC is open source or not, but you take the section of code that specifies gravity on say the moon and you copy it. You paste it in to your new code for the new planet. Now your new planet has the exact same gravity as the moon. If Mitch wrote GC as an open source project, he's probably got all the different sections commented and variables marked, so people can easily understand his code.
It would be made that instead of a hardcoded variable, a custom planet pulls some kind of player input and uses that as the planets gravity variable.
It would probably be a lot harder, but I think pretty neat to see implemented:
Some kind of genesis machine/rocket could be added to the game to handle the this. I think a genesis rocket would probably be better. You build a special rocket. The tier would decide the distance from the sun, which would decide which tired rocket is needed to get there. A tier 3 genesis rocket would mean a tier 3 rocket is needed to get there. Tier 2 needs tier 2, etc. There would be other parts added to the rocket that would dictate other planetary traits.
Loading the rocket with various forms of stone would affect what the planet is made out of. Ores would be a blacklisted item to prevent people from trying to create gold or diamond planets, which would be totally unbalanced.
The rocket would have say 3 or 4 (or more) slots. You fill those slots with a stack of material, and that would decide what the planet is made out of proportional to each stack size. The size of all the stacks would also affect the mass of the planet, which decides what the gravity is.