Just in case you don't know:
Make sure your GameCube is one that can take the component cable. The GameCubes released in the first year or so could. After that, they took out parts to reduce costs and later GameCubes couldn't take the component cable.
How do you tell? If you have a GameCube with two A/V ports on the back (one is the traditional SNES-like A/V port and the other one looks like a Wii A/V port (but they're different)), then you're good to go. If you just have the SNES-style A/V port you're out of luck.
Just didn't want anyone to waste time tracking this stuff down and buying it when they may not even be able to use it.