Try reading this: http://ftb.gamepedia.com/GregTech_5/Mining_and_Processing
Gregtech removes vanilla ore generation and uses its own. Try looking for HUGE veins of ore that usually measure 9 chunks or bigger. Iron can be smelted from Brown Limonite, Yellow Limonite, Banded Iron (found in limonite veins with malachite) and the Gregtech Iron ore (from copper veins). Diamond is found with graphite and coal at y-levels 5-20. Looking for specific veins may take forever but once you do, you're set for quite a while.