There are config files in place for gregtech. Make sure you use the correct version otherwise you will get issues. The version of ic2 will determine what gregtech you will need.
This part is VERY IMPORTANT. If you're trying to use the latest version of GregTech, it may be built against a different version of IC2 Exp than the one that's in the DW20 1.6.4 pack. You need to make sure you're getting the version of GregTech that's built for the version of IC2 Exp you're using.
At the moment he is up to 4.08n Greg only offers the latest version for each version of minecraft for download. Just found out the has a download link for the version Arakasiii mentioned. In the spoilers on the second page for Downloadlinks of OLDER versions spoiler he has 4.06i If you choose to use the latest version which is 4.08n at the moment you must update your IC² Experimental version to at least 374 According to the wiki change log here.
