I don't know Easyjet's situation in Gatwick but for what it's worth - We do often know where we are going to park before we land but we don't normally know before we take off - so it wouldn't be much use as far as parking your car the week before goes. Sometimes we don't find out ourselves until given the taxi instruction from ATC. Even then there are a huge amount of things that can change/upset this. Maybe there was a problem with the planned stand, maybe and aircraft is stuck on it with a slot or a tech problem or is just late, this only really has to happen to one aircraft/stand before there is a knock on effect whereby someone else has to go to a different stand (normally the one you are supposed to be on or so it always seems) and then you have to go somewhere else.