I am not doing any work whatsoever on a 1.12 version, for now. I do not know if any other developer is working on this, but I doubt it, there is not even a branch for it.
Currently I am looking at some of the issues in Galacticraft 4 including compatibility issues with other mods which are very time-consuming to solve, and also looking at completing some of the intended features for GC4 and a few things that micdoodle8 and I have planned in the pipeline.
I never understand the eagerness of players to have a mod in the very latest Minecraft version (e.g. 1.12). What difference does it make to you, between 1.11.2 and 1.12? All the mods you want to play likely exist in 1.11.2 - indeed they're probably more stable for 1.11.2.