PR is an option, but it is hard to make the PR for this due to many reasons.
1. Galacticraft only changes the behavior of Player.
Of course there are some alien mobs, but they are custom mobs by Galacticraft which can easily be modified (for Galacticraft).
2. Modifying Gravitation Effect is not that easy. For gravitation effect is hard-coded, and each entities got different methods for gravitation. Plus, there are custom entities added by mods which would have different mechanism of gravitation.
3. If this would made as a PR, it will contain so many changes. It would make the PR hard to accept.
If you think a PR will be hard to accept, who do you expect to do this? PR or a patch by one of the Forge Team, they would both contain the same amount of changes. You don't even need this in forge, use ASM or Reflection. I will say 1 thing. If you want this, you will most likely need to do it yourself and PR it.