Now then callum.
With gate starts its based on your position on the bike.
Start off by positioning your cranks in the right place. If youve ever seen a gate (which im guessing you have) you'll notice that theyre positioned at an angle. Set your cranks so that if you were to draw a line from the back of the gate to the front they would be flat. (most people line their cranks up with the chainstay, not a good idea as it ruins your balance). Next up is body position on the bike. i personally find it most useful to centre myself over the bottom bracket, you'll see some people lean right back over the rear wheel. But personally i think centring your body weight over the BB works best. Next up is your arms, wrists and hands. You want to keep your arms straight and curl your hands around the grips so that your wrist almost arc. It should feel like your a coil ready to spring. This is how you should stand in the gate.
http://www.pinkbike.com/photo/3856840/As the gate dropped you want to shift all your body weight thats centred over the BB forward. You want to do it FAST! as you do this you want to be pulling at the bars to really help force yourself forwards.
http://www.pinkbike.com/photo/3856859/When the gate drops dont do any of this wheely about 2 metres silly business that you see the world cups riders do. It doesnt make your gate any faster, its just the fact that their legs are so powerful as they crank down their producing so much power the front end lifts off. when you get more powerful youll end up doing it too.
thats the bear bones of a gate start. if you search on yetibikes theres a podcast with jared graves where he gives an absolutely amazing breakdown of a gate start