As the minecraft is developing, More and more mods come out with higher versions and better playability. With the stop of optifine in version 1.7.10, more and more Shaderpacks (Version 1.7.10) are no longer supported. Resourcepacks are developing as well. More high-quality and more HD real games are moving towards higher versions. Many players want to get a higher experience, but Gregtech is in the awkward position of low version.

because the game gets worst performance wise and changes extremely on how mods work. although i enjoy the water features add in 1.14 I dont feel GT6u needs to just to a higher version especially not 1.12, 1.12 is a disaster when it comes to performance. now have you tried Gregtech Classic Expansion? it is already in 1.12.