I just made a modpack with this mod. But I get a crash when galacticraft is installed if I create a new world. And the log says something about biome IDs. Will post the full log if needed.

Yes, post a full log. And don't make any public modpacks yet with it,, it's not even alpha. I can't guarantee that things will be compatible in the future. And by log I mean fml-latest-client or whatever it's named.

Are you as a player in the world when it's being generated? it looks a lot like GalactiCraft "bug" with the root cause being a vanilla bug. It happens with CubicChunks more frequently because world generation is slower, so there is much higher probability you will be in the world before any chunks are generated. That together with cubic chunks generating biomes more frequently for the same chunk makes the issue appear much more frequently.

Also looks like you are using the new launcher, which eats the crash logs and never shows them in the log. If it's still an issue, either figure out how to give me: latest.log, fml-client-latest.log and the crash log output from the launcher, or give me fml-client-latest.log when running with the old launcher.

Make another issue for that, I will see what can be done there. If it keeps saying 0%, then it is making progress, just very slow.

I will just test it myself...
You have a lot of error messages from interaction between JEI, BuildCraft and something else, and on top of that there is the crash from GalactiCraft. And you seem to be confusing a few different crashes. Once you are talking something about IDs, then I see crash in PlayerCubeMap (which probably is caused by the mod doing something strange)

But tha additional log spam other mods generate makes it harder to see what is actually wrong, anyway testing it myself because I don't see anything obvious that would cause it.

It's not the only GC issue. It still crashes if you try to do manh GC-specific things outside of 0-256 heigth range, even placing a torch may crash below y=0 for example on mars.

The original issue has been fixed both on my and galacticraft end. There isn't anything I can do to fix GalactCraft hardcoded heigtht limits.