Did a bit of digging and found this:
SCART is an industry standard for interconnecting audio-visual equipment, developed by the French company, Peritel. SCART stands for Syndicat francais des Constructeurs d'Appareils Radio et Television, meaning "Association of French Radio and Television Manufacturers". In Europe and other countries that use the PAL video standard, most TVs and VCRs come equipped with a SCART connector. The Peritel SCART connector (also known as Euro-connector) has 21 pins and provides for bi-directional connection of audio and composite video signals, uni-directional RGB video, plus control signals for television/VCR interaction.
"Most SCART cables available for Gamecube do NOT deliver an RGB signal but instead wire the normal AV composite signal to the SCART, so that you can use the Gamecube with a TV that has only SCART connectors. The only real Gamecube RGB Scart Cable comes from Nintendo, but this cable does not provide any additional audio outputs, so you only have your TV's speakers for sound, and cannot connect your Gamecube to a Surrond Sound Receiver. Looks like on the Gamecube, you can either have great picture or great sound, but not both. "
So why you did buy an unofficial scart cable you're basically getting a converter :0. You're not getting progessive scan :0.
Also you can buy unofficial VGA converters which can deliever 480P but that's another issue :0.