Your PAL Gamecube supports RGB through SCART so you can use a Retrogamingcables Gamecube RGB SCART cable. It is best to buy one without sync stripper. Even if you buy component cables for your Gamecube then your PAL games won’t support 480p.
For the Wii you should buy a component cable so you can enjoy 480p. Luckily those are not so expensive.