GAG is a Boot Manager program. It's loaded when the computer is turned on and allows you to choose the operating system you want to use.
Its main features are:

Allows boot of up to 9 different operating systems.
It can boot operating systems installed in primary and extended partitions on any available hard disk.
Can be installed from nearly all operating systems.
GAG doesn't need its own partition. It installs itself in the first track of the hard disk, wich is reserved for these kinds of programs. It can also be instaled on a floppy disk, without using the hard disk.