The most reliable place to confirm your terminal is your actual booking confirmation or boarding pass rather than memory or a general assumption about your airline, since allocations are not fixed forever and can shift between when you book a flight and when you travel. If you are at all unsure, checking your airline's confirmation email or the airport's own flight information a day or two before travel is worth the two minutes it takes, particularly if it has been a while since you last flew with that carrier from Gatwick.
Terminal changes do occasionally happen closer to the travel date, and if that happens to you after you have already booked your transfer, letting us know as soon as you find out is all that is needed — we update the driver's instructions and pick-up point at our end, so there is nothing further for you to arrange. This is exactly the kind of change worth calling about rather than emailing, since a same-day terminal switch needs the driver's plan updated before they set off, not after.