GIF
Stands for Graphics Interchange Format. GIF images are the most widely used graphic format on the web. GIF images display up to 256 colors.
Glow
A glow is the opposite of a shadow in that it creates a surrounding highlight of an image. A high radiance creates a soft, subtle glow and a low radiance creates a hard, bright glow, such as a neon glow.
Gradient
A gradient is a gradual transition of colors. Many metallic images are gradients. Web images that use gradient fills as a special effect should be saved in a JPEG rather than a GIF format.

Grayscale
An application of black ink (for print) or the color black (for the screen) that simulates a range of tones. Grayscale images have no hue (color). In print design, a grayscale graphic image appears to be black, white, and shades of gray, but it only uses a single color ink.