A standard RGB SCART lead does not exist , you can use composite or S-video, of course, but the picture quality isnt that good, and you may have problems with the colour.
It is possible however to use a combination of an official nintendo component lead, an s-video lead, and SCART lead, thse can be bought in some shops for around 60pounds or you can make one your self
you need the folowing
Nintendo Gamecube Official Component Cable or D-Terminal Cable *
2. Nintendo Gamecube Composite Video Cable or N64/SNES RGB Scart Cable **
3. 1 x 10k Ohm Resistor
4. Low wattage soldering iron (12W – 18W) with a fine tip (1mm chisel/pointed)
5. Thin gauge rosin core solder.
6. A spare scart plug with a full 21 pin (solderable) layout.
and the guide on how to do it is here: