Throw away all your old concepts of mining in Minecraft. Gregtech completely rewrites how ore generation works in this modpack.
First go read the "Ores" section of this wiki http://ftb.gamepedia.com/GregTech_5/Mining_and_Processing#Ores. Ignore the "Default Ore Mixes" section!
Right off the bat know this: Exploring cave systems and traditional strip mining is completely worthless and a massive waste of time.
Your first task is to identify the ore generation grid. As described in the wiki page, all ore generation happens in 3x3 chunk sections, even if the vein itself is smaller or larger than 3x3 chunks.
Any mining that you do, whether before or after you identify the grid should be done using vertical shafts. (the only exception being if you want to make a horizontal tunnel underground to connect each shaft together, but this tunnel is not for finding veins.)
The vertical shaft should be positioned in the center of whatever chunk you are mining in, you can choose how the shaft is structured, but I recommend a 2x2 block shaft so that it can be a staircase of sorts.
This image is a topdown map of a random world where the stone/dirt/etc has been removed leaving only ore veins and small ore blocks. The thin gray lines show the vanilla chunk grid. The red lines I have drawn ontop of the original image, to help show the 3x3 ore generation grid.
To identify this grid without removing all the stone/dirt/etc, pick a bunch of chunks and start mining a shaft in each one. Use the size of any veins you find and your map to help you identify the grid. Remember that all veins generate out from the center chunk of each 3x3 chunk section. However also note that some veins can be 2x3 or other rectangular shapes, so if you dont find a 1x1 or 3x3 vein right away, you may need to find more veins nearby, or use existing veins you have found already.
If you think you have figured out the grid, test it out and correct it if needed. Once you figure out the grid, or otherwise have a guess for how it is positioned in your world, you only need to mine 1 vertical shaft in the center-most chunk of each 3x3 chunk section of the grid. If you find a vein there, great, if not move onto the next 3x3 chunk section. This way you are gurenteed to find every vein, assuming a vein generated. As seen in the above image, some sections of the grid do not have any veins in them, this is completely normal.
Good luck and have fun! Note that this process may take at minimum a few hours, be prepared to mine for quite a while. If you have any questions please contact bigbass1997 on Discord.
TODO: This guide needs to be revised with new information about how to identify the ore grid without ever mining any shafts.