From the RedPower EAQ at www.eloraam.com:
"Somehow, you’ve managed to incorrectly install Forge. There are many ways to do this. You might have used a frontend program that told you that it conflicted with ModLoader, and then prevented the overwrite. Or you may have installed ModLoader on top of Forge."
Of course, provided that the rest of the game is working just fine, it's probably not a problem with Forge, so here are some quick fixes you can try relevant to modifying the FTB client:
Have you modified your game files in any way whatsoever except for changing GregTech's config? If you have, revert any changes that you've made and try again. If you haven't, revert the GregTech changes and try again (I'm not sure why that would change things, but who knows? If this doesn't work, change GregTech back how you like it.)
These are quick fixes that might work, whereas the more likely scenario is that the copy of Forge that you've received with your FTB client is somehow corrupted/broken, or you've installed ModLoader on top of it, which is both a no-no and redundant as Forge has ModLoader present inside it by default. Try force-updating (Options tab --> Force Update, then Launch in the FTB launcher.) If that doesn't work, delete everything in your mods directory (on Windows, C:/Users/YOURUSERNAMEHERE/AppData/Roaming/.feedthebeast/Ultimate/minecraft/mods) while keeping the config files, then launch FTB to redownload the files. Also make sure you've not moved anything from a folder where it was to another folder (e.g. moving RedPower from mods to coremods or moving Forge in the opposite way) and that you've not installed anything into your minecraft.jar such as OptiFine. These kind of mods should go into C:/Users/YOURUSERNAMEHERE/AppData/Roaming/.feedthebeast/Ultimate/instMods.