Yes safe. It's meant to do that anyways. BTW nowadays rcd detects by measuring the difference of current between live and neutral, if short somewhere the amps will be different hence will trip. The earth wire just helps to redirect leakage current and doesn't help in the detecting and tripping.

Mine swing type
Just use small plastic casing bought at shopee

All attachment is by 3M VHB double tape.

Remember to seal the casing door with black tape  to keep out moisture

And please use the magnet side at the gate locking mechanism.

As in my picture.
[attachmentid=11499010]

Actually, some people more daring, they just fit the sensors without any protection from the rain.

Here is the picture of the small box, but I dunno if it fits yours
[attachmentid=11499867]

I use black tape for both the devise and small box. No problem with rain.
Silicone I don't think will stick on the box material type.

Make sure to align the magnet and detector inside the box.
Example, my gate when closed is not really align, so you can stick the device in the box in such a way both are align. Quite difficult to explain to you.

And take note on the position below.

[attachmentid=11499868]

[attachmentid=11499869]

Happy DIY and do it on a cool day. Later get headache due to the hot sun cos you will need to do some adjustments. Buy the 3M VHB tape to do all the sticking. And black electrical tape to seal the sides

If your gate has like a raised portion for like the lock mechanism like this, you can put the sensor under it, and put electrical tape around the sides, i used it like that for 3 years with my Aqara sensor, i was actually surprised that the battery lasted that long tbh.

https://www.google.com/imgres?q=dc%20moto%2...ZAO4QM3oECG8QAA