For a long-haul flight from London Gatwick, is arriving 2½ hours in advance enough?

Each airline has different rules for checking in, so you need to confirm the final time that they allow for this, for long haul flights this is often more than short haul.

For Gatwick at that time in the morning I would think that you should be OK with that amount of time - you may have to Queue for the check in, and then at Gatwick security the lines can be be very busy early in the morning. (I queued for 35 mins earlier this year). Depending on your gate you may have a long walk to get to the plane.

At Gatwick I would allow 30/40 mins for checking in, 30/40mins for security, and a potential 30 min walk to the gate.

If you live close to the airport some airlines offer a 'night before' check in / bag drop, which can save time on the day.

http://www.gatwickairport.com/at-the-airport/flying-out/check-in/day-before-check-in/

That should be more than just adequate. I have taken a passenger to LGW and she caught her international flight despite arriving at the (by then closed and near deserted) check-in counter less than ten minutes before scheduled departure (pre 911!). But in this case I would recommend the earlier train. The problem is that others, perhaps for similar reasons, may also be “cutting it fine” and if all 333 Economy passengers (assuming a full A330-200 - I would guess the flight will be near capacity in that cabin) do so, check-in staff may struggle to process everybody in time.

Air Transat counters open 4 hours prior to the scheduled time of boarding and this almost states that, other than for USA, 3 hours is sufficient:

Passengers travelling to the U.S. require additional processing time at the airport for security, as well as for clearing U.S. Customs and Immigration. It is imperative that you arrive for check-in at least 3 hours prior to your flight.

So something under 3 hours should serve for long-haul other than to USA. But unless the additional procedures for USA take ½ hour or more, 2½ hours for other long haul seems to be about the limit for safety.

The critical point is check-in, subsequent procedures are now quite slick at LGW and once your luggage has been dropped security concerns give you a measure of ‘protection’.

Check-in closing times

Close 1 hour prior to the time of departure. All Air Transat check-in counters close 1 hour prior to the time of departure. Passengers arriving for check-in after that time may be denied boarding.

These are not typical but bear in mind stories such as recounted here.

Personally, I would expect to sleep better with an hour less but in the knowledge that I am very unlikely to miss my long-haul flight than to have an hour more in bed though not be quite certain I will catch the flight I have booked. In any case, other than topping up with alcohol, arriving on board tired out is about the most effective way to make the time (10 hours?) for a long-haul flight seem to pass less slowly. Given the time difference this may also help with adjusting your biological clock. And, the train service may be the weakest link in your travel plans. With the service hourly miss one train for whatever reason and you lose all margin for error.

In due course I would be interested to know what you decided and how you fared.

If travelling Business, forget all the above, you have plenty of time.