GregTech is an add-on to IndustrialCraft2 which increases the IC2 techtree with 2 tiers. Many recipes of the lower tiers are re-balanced, like the Gregtech recipes for
the Macerator, Mining Drill, Mass Fabricator as well as some method of powering all of these. This greatly increases the difficulty of gathering/creating more rare materials such as UU-Matter and Iridium. In order to reach the upper tiers you will need to stripmine dozens to hundreds of chunks and you will need to find The End to exploit its unique resources.
GregTech also improves compatibility between IC2 and other mods, such as RedPower 2 and Buildcraft 3. GregTech adds several dozen more machines and multi-block structures.
Each Nugget is equivalent to 1/9 of its corresponding ingot. If 9 Nuggets of the same type are acquired, they can be combined in a crafting grid in the same method as combining Gold Nuggets in vanilla Minecraft to acquire one Gold ingot.