Short description for creating GifMasks:
In principle you can use any program which supports a transparency color for GIF files (Photoshop, Paintshop, Gimp...). Otherwise you can create a 'normal' 24 bit picture (BMP-file) first (even with Windows Paint) and define the background color as transparent. This can be done very simply with IrfanView or GFDGifTool.
IrfanView: Open your BMP file, use save (or save as) and set the file type to GIF. Make sure that 'Show option dialog' is marked and within the options dialog: 'Save transparency color' and 'Choose transparency color during saving' are marked too. Select file name and path and click 'Save'. Click in the 'Choose transparency color' dialog on any point in the background.
GFDGifTool: Open your BMP file with 'Add BMP file to list...'. In the right preview frame click on any point in the background (thus the transparency color is set and "Make transparent" is activated) and save the GIF file with 'Save BMP as Gif'.
To use your GifMask (or any downloaded Gifmasks from other sources) in GfD, you must put then in the GifMask directory within your GfD installation directory (usually this is C:\Program Files\GUI for dvdauthor\GifMask). Since GfD version 1.05 you may also put your GifMask files in an own GifMask directory. This directory is configured in the Videos+Image settings and saved within the project file.