Summary

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 on a 350MHz iMac G3.

Workaround

In order to install GarageBand 1.0 on unsupported G3 systems, follow these instructions.

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 and double click it.
7. Follow the installation steps.

(Note: GarageBand 1.0 does not have the Keyboard Piano, it's a novel feature of GarageBand 2.0)

Updates

2021-04-12: Grammatical corrections.