You might try here first:
http://www.moto-master.com/default.php
search by bike name. There is a USA
dealer, if you are in the States.
You might look up motostrano.com and
give them a call.
GoFasters may also be able to get you
a real GasGas brand front rotor as well
as a caliper relocation bracket.
As far as I know. The SuperMoto wheel
hubs are the same as Enduro wheel hubs.
If you have not found it yet. Supermotojunkie.com
has lots of general SuperMoto information.
Not a lot on GasGas though.
Here is my set up for a 1998 EC250SM with the
Marzocchi 50mm forks and Brembo caliper.
Yours is most likely going to be different.
Good luck, Jeff