The Sky Dimension is in GravityCraft 1.2.5. I'd like to think that it will still be available in GC 1.3 if there is a GC 1.3 since the SkyRealm isn't actually in the vanilla 1.2.5 code anymore anyway, so GC must be supplying it.
Once I'm able to afford classes on the LearnToMod website, I'll try reviving it myself. Reviving this mod is a priority of mine, but I won't rush into things. I'll make different mods and such first so I can get used to the mod creation process, and then eventually start work on this masterpiece. (With permission of course.)
I agree, forget about necro posting.. this needs to be revived without a doubt. The only issue with that is it needs to be picked up by someone who knows what their doing and most have their own mods to worry about so no one is going to get their hopes up.
Its been 6 years since it died so it's obvious patiently waiting for a good coder to pick this up isn't going to work at this point.
On another note whoever picks GravityCraft up would only have to port the layer features( aka the Mine Up part of it ), won't even have to port the gravity since this exists: https://minecraft.curseforge.com/projects/up-and-down-and-all-around