I have stayed at the Gatwick Hilton a few times over the past few years. Overall it is good, being hugely convenient for the South Terminal (a short walk away). The rooms are modern and comfortable and there are plenty of choices of places to eat (bars, restaurants etc.) Also I used the gym this time, which was also good.
My only issue is that it is a busy hotel. If you happen to be checking in at the same time as the Qatar crew arrive (for example), then it can take quite some time. Also the sports bar can become full. There can be quite a bit of corridor noise due to the number of rooms and nature of the hotel.
That said, for the typical price and convenience it ticks all the boxes.
I prefer the Sofitel at the North Terminal, but that is priced accordingly (with less eating options). The Premier Inn is slightly cheaper, but is far busier and often there is a huge wait to get food.