I got two/ three answers for you.

1. Get a HD Retrovision snes cable.

2. Use this and this

3. This is the most complicated option but it will work. And it will look amazing

Order this

Open your Pal gamecube

follow this pinout

You can lift the legs and make the component signal come out of the rgb locations

or

You can tap/ solder leads (From the ordered board rgb Csync locations) to the underside of the multi out (RGB and Csync) then add a component pigtail out the side air vent of the GC. Youll just need to clip it at the Dsub connector and just use the female leads.