I assume that you took my advice and made sure that you were using the latest versions of Processing (2), G4P (3.2.4) and GUI Builder (2.2.2) - if not please state which versions you are using.

Although I don't have Windows 8 I don't believe this is the problem.

I have looked at the source code for GUI Builder and it seems that the error appears to be generated by Processing in the Sketch.java addFile() method. The actual source code for this is

1. // in case the user is "adding" the code in an attempt
2. // to update the sketch's tabs
3. if (!sourceFile.equals(destFile)) {
4.   try {
5.     Base.copyFile(sourceFile, destFile);
6.   }
7.   catch (IOException e) {
8.     Base.showWarning("Error adding file",
9.     "Could not add '" + filename + "' to the sketch.", e);
10.     return false;
11.   }
12. }

It appears that it can't copy the gui.pde file from the tools data folder. (line 5)

It almost certainly means that you have not installed the tool correctly. Make sure that you install ALL the files from the download not just the library jar file. Follow the instructions here and when done examine your tool folder and make sure everything is there especially thethere will be a folder called data inside the tool folder.