Create copy of Gravship scenario (feel free to disable pursuing mechanoids if you want) and save it.
Go to %userprofile%\appdata\locallow\Ludeon Studios\RimWorld by Ludeon Studios\Scenarios
in explorer and open your saved scenario file with prefered text editor (eg. VScode).
Find part that looks like this:
<li Class="ScenPart_ForcedMap">
<def>ForcedMap</def>
<mapGenerator>OrbitalRelay</mapGenerator>
<layerDef>Orbit</layerDef>
</li>
and replace OrbitalRelay
with AsteroidBasic
.
Save your file, then go into Rimworld, and now if you Load your saved scenario when creating new game it will start you on an Asteroid :)