GAG (initials, in spanish, of Graphical Boot Manager) 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.

GAG Graphical Boot Manager Features
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.
It has a timer to boot a default operating system (selectable by the user).
The configuration menu can be protected with a password.
The program works in graphic mode (needs a VGA or better graphic card), and has a lot of icons.
Hides the primary partitions which allows the user to have instaled more than one DOS and/or Windows in the same hard disk.
Allows a password to be put on each operating system, denying access to non-authorized people.
Allows the boot manager text to be translated to all languages.
Can exchange disk drives, allowing to boot from the second, third... hard disk operating systems such as MS-DOS.
Has the SafeBoot system, that allows to boot your hard disk even if GAG is accidentally overwrited.
Supports a great variety of keyboards (QWERTY, AZERTY, QWERTZ and DVORAK keyboards).
Fully support for hard disks up to 4 terabytes (4096 gigabytes).
Full version and free software (distributed under GPL licence, with source code)