(ANSA) - ROME, SEP 7 - Supermarket giants GS and Auchan (now Margherita) are among those being probed in a 1.8 billion euro tax fraud case, Milan judges said Wednesday.     Nine people have been placed under house arrest on charges of issuing 1.8 billion euros in false invoices and evading VAT to the tune of 260 million euros, judicial sources said.     Some 33.8 million euros were seized from GS, which is controlled by Carrefour, and 26.2 million from Auchan.     Carrefour opened an internal probe and said it was ready to collaborate with prosecutors.     Among those placed under investigation in the probe are Gerard Lavinay and Stéphane Coum, respectively ex managing director and ex operational director of Carrefour Italia.     Wholesale giant Miti is also being probed.     All those probed deny wrongdoing.     Auchan's former buyer Gianpietro Racagni is alleged to have received in compensation for the alleged fraud two electric bikes, trips abroad, other gifts and soccer stadium tickets, police said. (ANSA).