This post box is suitable for palisade fencing.

Yes, this post box will be compatible with your gate railing because the attachment is adjustable.

The post box is not difficult to install, but it will take two people. One to hold the box firmly against the gate while the other bolts through the post into the fixing brackets on the other side of the gate. The images above show the brackets.

The fixings supplied with this box are designed for railings. It would require some customization of the fence on your behalf to allow the box to fit flush for this to be suitable.

Unfortunately post boxes are not immune to theft and people potentially trying to hook mail out. If there's only a handful of mail in the bottom of the box and you emptied this daily you significantly reduce the chance of people trying to get any mail out, although this isn't impossible unfortunately.

It will depend on the size of the newspaper or magazine to be posted.
The size of the letter slot is 235mm wide and 32mm deep.