Not saying this is very necessary but maybe you consider making new, small mod.
This mod should do a simple thing, get list of mobs ID's from config file, and apply atribute so mobs don't suffocate in Galacticraft no-air space.
Here is API for mobs.
LINK

Not sure if this helps, but here is a mod that takes vanila mobs and add new AI to them
LINK
Maybe you can figgure out how to inject this atribute

Here is example of atribute implementation
LINK
You add:
import micdoodle8.mods.galacticraft.api.entity.IEntityBreathable;
and
public class EntityEvolvedWitch extends EntityWitch implements IEntityBreathable
And the rest... unfortunately i don't know how to make this mod, maybe you can figure it out.

Ofcourse this mod will be yours, no need to mention me. All i need is mod for my modpack.

But if you have no time for this, no problem. I just wanted to add some non-Galacticraft mobs on a moon. Exept they suffocate and die.
And not only me mentioned this request LINK but left un-answered.
Thanks!

So it looks like the latter option might be best. You'd need to basically make your other mob-mods into an addon for Galacticraft and have them implement IBreathable (which may be a pain if you have quite a few mods).

It's likely there is an easier/hackier way to do this, but unfortunately I'm not good enough at modding yet to do it. I'd recommend joining one of the many modding Discord channels and asking them - lots of people here enjoy a challenge 😎

Hi,

I'd have to do a bit of investigation into this, but if it's a lot of work I may not have the time. Is it just vanilla mobs you want to breathe in Galacticraft?

Quite the opposite. I want to tweak modded mobs. But by your question I start to suspect there is a problem with injecting into mobs from different mods. I don't know how mods work. So I think you shouldn't even bother. Maybe contacting with mod authors would be better thing to do. Just some of them long time inactive.

Well in that case probably the simplest thing to do is clone the Galacticraft repository and just change their IBreathable interface a bit. You could then have your own custom Galacticraft mod, although that would mean you would have to update it if you wanted a new version of Galacticraft (not a problem if you don't update Galacticraft often).

Without having looked to deeply into it, I think the alternative is compiling your modded mobs mod against Galacticraft and have them implement the IBreathable interface.

I'll take a look tonight and let you know if it's straightforward 👍