This information applies to Mac OS X, GIMP 2.2
Sometimes while working on a project you may want to save the settings you used in a Levels or other dialog to use on other images in the project. When that project ends, you will probably want to delete those saved settings.
I found myself in this situation recently, but I was unable to find any documentation explaining how to delete these saved Levels settings. I was eventually able to determine these settings were stored in a folder in /home/.gimp-2.2. I then learned that since this folder name begins with a dot, it is hidden by OS X.
Next, I had to figure out how to access this hidden folder. I found the following scripts which run from the Terminal. Type this in the Terminal to show all hidden folders and files,
defaults write com.apple.finder AppleShowAllFiles TRUE
Then, open Finder from the Dock and navigate to the .gimp-2.2 folder. Open it and delete the saved profiles.
You'll likely want to restore file/folder hiding. Run the following,
defaults write com.apple.finder AppleShowAllFiles FALSE
A note of warning, be careful when modifying hidden things. Backup first.
Hopefully, future versions of GIMP will have a simpler way to delete these no longer needed files.