My 4200 doesn't, and I'm surprised if the GS has them.
It certainly wasn't a deal breaker for me, trying to find any high performance coupe with a big enough rear seat for kids is hard enough, and with the Maser adults can sit quite well too.
My 2 and 6 year olds both have child seats, my daughter a booster now, but my son has an isofix seat for when in my daily runner, which happily fixes in the back of the Maser using the seat belt. It helps if you pack up the rear seat depression so the seat is flat. You can buy or have made up a suitable cushion instead of towels if you wish!
Cheers
Mike