I don't have my abode system yet, but it is my understanding that third party devices can only be used for home automation. So there is no third party sensor that could trigger your alarm (only trigger home automations like turn on a light).

So far the problem seems to be just finding third party contact sensors that will pair and have their state recognized. I considered trying the Aeon Labs recessed sensor (which is on the supported list) thinking that maybe it would stay dry recessed in the fence post. But looking at it, it's pretty open and I suspect that water would wick into it pretty quickly.

I don't alarm on the gates, I only send a notification. Which means that right now once I get a working sensor I'll have to rely on IFTTT for this. Hopefully they'll add a notification action in the new automation engine.

I imagine the abode door/window sensor would work on a gate. A waterproof case like this should fit the larger piece and not look too ridiculous: https://www.amazon.com/dp/B0007PENQK/ref=twister_B0178DQRVW?_encoding=UTF8&psc=1

This is the route I'm taking. I've ordered a couple of waterproof project boxes and some powerful dry contact switches intended for gates. I'm hoping that I can put the Abode sensor in the box and still trigger it as-is. I've ordered heavy duty gate sensors in hopes that the include what I expect will be stronger magnets.

If that doesn't work I'll mount the Abode sensor in the box and connect the garage contact sensors across the Abode reed switch.

I'll update on the results.