Just doing a little search this one looks pretty nice and its made to be outdoors: https://a.co/d/9RBtqJP
Wiring, it is just just two wires. This particular one actually has 3 wires so you can wire it as NC or NO (normally closed or normally open). I think you would want it wired NO, so when the magnet makes contact it reports closed. The ZEN17 can be configured to reverse it anyway so it doesnt matter a whole lot.
To clarify, this is NOT a smart device, it is just a contact sensor. But you can wire it to a smart device. If you have a power source there already and need a relay to open/close the gate then the ZEN16 or ZEN17 plus a wired sensor is your best option IMO. No batteries to deal with on the sensor that way. The ZEN17/16 can even be direct wired to a 5v DC power source.
They are Z-wave, so range could be a problem. If you have a C8 hub though they are now being sold as Long Range models so you can use that. The device you have now I think is Zigbee which typically gets a little better range than standard Z-wave.
Fully configurable using my drivers as well: [DRIVER] Zooz Relays Advanced (ZEN16, ZEN17, ZEN51, ZEN52) (if you do want to purchase I have an affiliate link on that post you can use)
Honestly not recommending just due to that though, it is a unique device I don't think there are many others if any out there, high quality and reliable. If I thought there was something else better I would tell you.
Oh... one last thing, if you do not have a waterproof location to hold the ZEN relay you will also want to find an outdoor rated "project box" to put that inside of and all the wiring.
