GAMGI uses the file extension to automatically determine the file format. 
To export a standard .tiff file of the whole graphic area of a top window, 
just give the filename the extension 
.tiff or 
.tif.
The command line application 
pnmtotiff, widely available and 
usually installed by default in most Linux distributions, must be 
installed, otherwise GAMGI will be unable to export a .tiff file.
This a large-size, high-quality, widespread format, which is particularly 
suitable to distribute among systems that might not support .png files very 
well.