https://www.sportsmaserati.com/index.php?search/749623/&q=child+seat&c[title_only]=1&o=date
Have a read above.
I've had a couple of seats in both my GTs, older models now, so not available an more, but most will fit OK.
If your planning on leaving it in (as I do) I'd recommend getting an isofix one, with the leg that goes down to the floor rather than the tether. That way the support is on the leg, and it takes the weight off the lower cushion, on which the leather will get dented, no matter how careful you are.
I've actually removed the lower cushion on my current one, as I didn't want it dented like my previous one. The Leg type seat enables you to do this.