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