To update your GC Dual from firmware 3.0d to 3.1, you'll need to follow this update process.
Here's a step-by-step guide: 

? What You’ll Need:

  • A GameCube console with the GC Dual mod installed
  • An SD card (FAT32 formatted)
  • A way to launch homebrew on your GameCube (e.g. Swiss via SD2SP2, SD Gecko, or another loader)
  • A GameCube controller

? Step 1: Download the Firmware Update

  1. Visit the GC Dual firmware directory on Dan’s Projects:
    https://web.archive.org/web/20240722191354/http://www.dansprojects.com/firmware/GCDual/

  2. Download the file named:
    GCDual_CDK3404_3.1.zip (or matching your GC Dual chip version)
  3. Inside the ZIP archive, locate the file:
    GCVideo-gcd-3.1-update.dol (or similar name)

? Step 2: Prepare the SD Card

  1. Format your SD card to FAT32 if it isn’t already.
  2. Place the update .dol file onto the root of the SD card or in a folder like /apps/gcdualupdate.

? Step 3: Boot Swiss & Launch the Update

  1. Boot your GameCube using Swiss (via SD2SP2 or SD Gecko).
  2. Navigate to the update .dol file on the SD card.
  3. Run the updater using Swiss.
  4. Follow the on-screen instructions. It should detect the GC Dual and prompt you to update.

⚠️ Important Notes:

  • Do not power off your console during the update.
  • The screen may go black briefly—this is normal.
  • Once completed, the updater will typically say “Update Complete” and may ask you to power cycle the system.

✅ Step 4: Confirm the Update

  1. Reboot your GameCube.
  2. Hold L + R + X + Y on boot to enter the GC Dual settings menu.
  3. Verify that the version in the top corner reads 3.1.