make sure they can not use a pry bar. to bust open the lock. i see what looks like a pin holder in the middle of the gates just above the lock.
as far as motion detectors that take pictures. not worth your time. they are low resolution cameras. and they also give off a flash. flash = someone going over to camera and stealing the camera to. and night vision is not worth it.
your best bet, if you want to nail the thieves. is to get good lighting, and then setup a regular camera. that can see within the range of of the light bulbs you use.
if you have power and telephone line already ran to building site. you might be able to pick up some motion detectors. and a cheap security system. that calls your cell phone. then you could make choice of contacting police or not.
perhaps getting a cheap nightvision camera. and run cord like it is being installed in the given trench near the gate or on a gate pole, like it is a permanent camera. and posting signs. perhaps even just getting a intercom box. but not wiring it up, just hang it on the gate pole.
if ya have a enclosed trailer. put some fake cameras on it, with a piece of wire going through trailer. making it look like you have a recording device setup inside. with a wheel lock on a tire.
if it is teens, they have some ballz on them. they will double check things.