Gateway
A computer system for exchanging information across incompatible networks that use different protocols. For example, many commercial services have e-mail gateways for sending messages to Internet addresses.

GIF - Graphic Interchange Format
A common format for image files, especially suitable for images containing large areas of the same color.

Gigabyte
1000 or 1024 Megabytes.

Guest Book
A simple guest book allows visitors to leave their name and a brief message from/on your site.

GUI - Graphical User Interface (goo-ey)
This term refers to a software front-end application meant to provide an attractive and easy-to-use interface between a computer user and an application.