The formula for French Excel:
=CONCATENER(
DECHEX(ALEA.ENTRE.BORNES(0;4294967295);8);"-";
DECHEX(ALEA.ENTRE.BORNES(0;42949);4);"-";
DECHEX(ALEA.ENTRE.BORNES(0;42949);4);"-";
DECHEX(ALEA.ENTRE.BORNES(0;42949);4);"-";
DECHEX(ALEA.ENTRE.BORNES(0;4294967295);8);
DECHEX(ALEA.ENTRE.BORNES(0;42949);4))
As noted by Josh M, this does not provide a compliant GUID however, but this works well for my current need.