wow, Brendan, this is a bit of a poser.

First question: Is your kitty micro chipped? This is important especially if there is any possibility of him getting out of your garden.

Is there a possibility of constructing a solid out door run, or enclosure that Harry could be in, and which Gus would be unable to get into?

One of our own Catness members, who is also an experienced cat rescuer is Beth Bach. She might be able to give you some helpful hints. Tag her, Erika Burkhalter and Deborah Camp in your story.