GregTech CE Unofficial (GTCEu) is a 1.12.2 mod which aims to improve upon GTCE to provide the ultimate 1.12.2 GT experience.  

A (very, VERY) brief list of some new features:

• Greatly improved performance and optimization from GTCE. Most users should enjoy improved RAM usage, FPS and TPS
• Fixed many GTCE bugs (invisible blocks, infinite amps)
• Improved player experience and QoL. Example: Different stone types of ores don't clog player inventories
• Completely reworked and improved fluid pipes and energy cables
• New item pipe system with huge throughputs and configurable logistics
• World generation improvements. Better performance, rebalanced ores. New virtual fluid veins for harvesting
• New content integrated from Shadows of Greg, Gregicality, GT5 Unofficial, TecTech, Nomifactory, GT New Horizons, GT IMPACT and more
• Fancy rendering effects including emissive textures and bloom
• Content added at every single tier of the game. No stone left unturned, this is a completely different GregTech experience from GTCE

And many, many, many more things...

Check out the Changelog for a full list of changes in version 2.0

GregTech CE: Unofficial requires CodeChickenLib.