I don't think that there is a Marriott at Gatwick... There is a Renaissance at Gatwick (Renaissance is part of the Marriott family of hotels...).
If you are talking about the Renaissance, then we stayed there a few times (most recently 2002, I believe) and thought that it was a nice hotel. BUT I have heard that it has gone downhill a LOT and have heard some bad things about it recently... so I personally would avoid it (shame, because it was very nice).
The Gatwick Holiday Inn is supposed to be nice and I can vouch for the Sofitel (stayed there earlier last month).
Boo