Thats a great solution ! I would take it a step further and drill through the back of the box to create 2 holes. Then use self tapping screws through those holes and into the metal gate uprights.
Reasoning is that the 3 M tape will degrade over time and the box will eventually fall off.
I need to do this project as I too have a dog. Which open/close sensor is this @slagle?
FWIW, Iāve had a regular ST Multi outside for over a year on a gate. I went the saran wrap route, added a layer of electrical tape to attach the multi to the fence, and it has been fine so far. And it was under about 2 feet of snow for most of February.
Thanks. I just ordered one on Amazon. I live in NY and am concerned with temperatures but youāve convinced me to try it. This will also tell me when my landscapes come 
NY winters can be rough, but this last winter was pretty bad here in the mid-Atlantic and mine survived. Iām more concerned that cold will deplete the battery. (Although Iāve had a Multi inside my fridge for at least 2 years at about 40 degrees and itās only just now at 13% life.
The Ecolink open/close detectors are cheaper, if you donāt want to risk the more expensive ST multi. The Ecolinks donāt provide all the data (like acceleration and orientation, IIRC) but it seems like you donāt need those features in this application.
Yep, our multi should do the trick too. Of course putting something like this outside always has risk. But, I have had mine outside for 2 years now.
How do you guys affix it to pvc fence? Outdoor rated double sided tape?
Duct tape. (I donāt leave home without it.) Although this is a metal chain link fence. It isnāt the prettiest solution, but both the fence and tape are black, so it somewhat blends in.
I was lucky. I was able to remove the cap of my vinyl fence post and put the battery part of the sensor inside the post and the magnet part on the outside of the gate. Works great through the vinyl and itās protected from the elements.