I found out what the .gef files are for in the iPronto Edit Help.

Gallery
The Gallery is a very efficient tool to design buttons on a page.
The look of a button in your configuration file depends on the bitmap assigned to the button. The Gallery offers you an extensive series of bitmaps and buttons, grouped in several categories. You can easily browse through the list of bitmaps and buttons. Bitmaps are displayed with a preview and their dimensions. Buttons are displayed with a preview, their dimensions and their label.
You can personalize the Bitmap Gallery and the Button Gallery by creating new categories, importing new bitmaps and buttons and removing categories, bitmaps or buttons from the Gallery.
You can also import and export a Gallery, containing bitmaps and buttons, to exchange bitmaps and buttons with other iPronto users. The iPronto bitmaps and buttons are stored in a Gallery Export File or '.gef' file.

Export a Gallery
From the File menu, select Export Gallery.
The 'Save as' window appears.

Browse to the location where you want to store the Gallery.

Click Save.
The bitmaps and buttons in the Gallery are saved in a Gallery Export File or '.gef' file.
-or-
Click Cancel.
The 'Save as' window closes without exporting the bitmaps and buttons in the iProntoEdit Gallery.


Import a Gallery
From the File menu, select Import Gallery.
The 'Open' window appears.

Browse to the location where the Gallery is stored.

Select the Gallery you want to import.

Click Open.
A message window appears:

Click 'Merge' to merge the new Gallery with the existing one.

Click 'Replace' to replace the existing Gallery with the new Gallery.

The bitmaps and the buttons in the '.gef' file are added to the iProntoEdit Gallery.
-or-
Click Cancel.
The message window closes without importing the Gallery.