If you don't need your suitcase, check it in when you arrive. Then go to the Hilton Hotel. Amy's bar is open until 2pm. Then, if they do chuck you out, go to M&S back in the terminal (open 24hrs) and grab some food. Go back to the Hilton, find a quiet corner and "read". I think the shops open around 5.30 in Gatwick themselves, but you should be able to clear security pretty early.