My first question is do you need to have the gate locked on both sides? Right now it looks like you have a double cylinder lock where you would have to use the key from each side. That’s often needed if you’re concerned about someone jumping over the fence and then being able to unlock it from inside, but I just want to be sure what the project parameters are.
Also, what’s the weather like where you live? In particular, how cold does it get in the winter and how hot does it get in the summer?
Finally, since there are now regional versions of the SmartThings hub, what country do you live in? That can affect available devices.
I am not aware of any battery powered lock for outside but you may be able to use regular electric/magnetic gate lock together with a z-wave relay. You will need a power connection for this though.
I seem to recall seeing a similar item possible attributed to Aeon Labs but cannot find that page again, nor is it listed on Aeon Labs own site. I suspect it may never have shipped for real.
Even if it become available it might be US only and not suitable for Europe.
The make would be Econet, they manufacture their own devices.
It looks like that particular device might not yet have come to market, I can’t find a certification statement for it and it’s mentioned in some 2014 releases as “in development.” You could write Econet and ask.
BTW, there was just a new project mentioned the other day which is using an IBeacon with an antenna for an exterior gate lock and integrating it with SmartThings. So if the OP is still looking for a solution, this might be worth exploring. I don’t know what the costs are on it, though.