Take a look at this line in Galacticraft core.conf:
I:biomeIDBase=102
You can change that 102 there to something else. You'll need to figure out a biome ID which doesn't cause conflicts, Galacticraft willl use around 5 biome IDs in sequence starting from that number, like 102-106.
Alternatively if that doesn't solve it you can use another mod named BiomeTweaker, that should be compatible with all recent versions of Galacticraft, if in doubt make sure to use the latest versions of both mods.