Apple said a long time ago in 2004 that GarageBand can only be installed with the iLife DVD in a iMac G3 with 600MHz and a DVD Slot-loading drive.

GarageBand 1.0 is unofficially installable even in an iMac G3 of 350MHz.

To install GarageBand 1.0

1. Make a DMG file of the iLife DVD using the Disk Utility, or download "iLife04.dmg" from the internet.
2. Put the DMG file into an USB stick or in a External Hard Drive.
3. Turn on your iMac G3.
4. Insert the USB device in it and open the DMG file. Skip check if wanted. An iLife Install DVD icon will appear in the Desktop.
5. Select the "Install iLife" package with a right click and select "Show Package Contents".
6. Locate the "Install GarageBand.pkg" file in a folder (I don't remember what is the name) and double click it.
7. Follow the installation steps.

(Note: GarageBand 1.0 does not feature the Keyboard Piano, that appears in GarageBand 2.

- Nicolas Bahamondes