Rupp - I think it depends on the design and structure of the gate itself. While I can definitely see situations where wind might move a gate quite a lot, with careful sensor placement combined with the use or rare earth magnets, hopefully a normal wireless sensor would work.
It probably means the sensor needs to be place near the latch mechanism. This provides a firm anchor point for the gate - one that hopefully doesn't allow it to move more than 1/2-1" or more without someone physically opening the gate. Using rare earth magnets would allow for a greater gap between the sensor and the magnet before opening the contact.
EDIT - Actually since Gopsu specifically mentioned a metal fence, I suspect the gate portion may sit too far from the fence to provide a contact like that. In other words, I suspect the gate and fence probably sit 1-2" apart from each other. In that case, I think you could put the sensor in the fence portion right where the gate latch would close around the metal fence pipe. Then place the magnet in the gate latch itself rather than on the gate. So when the latch is closed around the metal fence pipe, it would close the sensor contact. Raise the gate latch, and it would break the contact - whether the gate was actually opened or not. That is probably better anyway. Then you know if the latch is closed or not.
Gopsu - since you already own a wireless receiver for the system, I think getting a wireless contact that works with that system is going to be the cheapest solution. There are other solutions (like using some x10 wireless contacts like the ds10a and a WRG800 wireless receiver), but you generally would have to buy some sort of wireless receiver for any system to work. Since you already own one, it makes sense to use it instead of reinventing the wheel. Just my 2 cents.
