This is caused by one of two things. Either RedPower has an ID Conflict or GregTech has a problem with the folder path in which you have the mod pack installed. If GregTech is installed and detects that you have either Tekkit or Technic in your folder directory to the mod pack it will crash with this error as well.
The solution depends on which is causing the error. If it is GregTech then you need to reinstall without either of those words in the folder path. I suggest using this
guide to reinstall the recommended way. If it is RedPower follow this
guide to solve your ID Conflicts.
