till not sure what type of gate you are looking for, went thru my Japan pictures, didn't really have any good pictures of garden gates. Lots of temple and castle gates, but not garden. This is what I found, large images to show more construction detail.
This is what I imagine you are looking for:
I have many temple gate pictures, this is from Ginkakuji in Kyoto:
As for garden walls, this shows the construction of a wall in Kumamoto Castle, upper class garden walls are built the same way:
As for joinery details, not free, but I have and like this book: