.gpk files considerably speed up file re-loading. That's why it's a standard with WaveLab to create them, this cannot be surpressed in version 6 and older.

If you want these files to be deleted after working in WaveLab you can configure this by checking Options -> Preferences -> File -> "Delete peak files when closing audio files". WaveLab needs to recreate a new peak file when reopening the wave file.

Since WaveLab 6 audio peak files and memory files can be stored in an independent folder (Options/Preferences/Wave edit).

Since WaveLab 7 this preference can be found here:
Options/Audio file editing preferences/Editing

• Save view settings in companion file
• Save in an independent folder

Since WaveLab 9 this preference can be found here:
File -> Preferences -> Audio Files -> Editing

• Save view settings in companion file
• Save in an independent folder