How to Keep Cats out my Garden when i have an Outdoor Cat

I have a neutered 5 year old mog called Gus. Gus does have access outside when myself or my partner is at home, we don't have a cat flap as we have a glass back door.
I have noticed quite a few cats in the neighbourhood and  on more than one occasion they have come into the house, I'm worried that this may stress poor old Gus out.

Recently I have noticed that he sits out side for long periods of time just watching, and  i think this may be looking out for other cats.
Does anyone know any ways of deterring or keeping these cats out my Garden but still allowing my cat to go out? I'm quite concerned about him being stressed!

Thanks

Does he look relaxed?  When Eclipse thinks another cat is around he is crouched down like he's hunting a bug.  He also roams the yard smelling the ground where the other cat has been.

He doesn't look agitated but i wouldn't say its a relaxed posture, and he doesn't move at all he's doing it either.

If you find a solution let me know.  I care for 3 ferals on my property.  They rarely leave my yard.  My neighbors cat comes to my yard each day and is now attacking all 3 of my cats.  I have informed her.  She now keeps him in at night, but says it's impossible to keep him inside 24 hours a day.  I try to watch for him and shoo him away, but he still comes back onto my deck stalking my cats. 

I have wracked my brain for a solution.

I'm sure it is upsetting and annoying when your neighbor's cat comes onto your property and it is your cat that becomes upset.

One solution might be a catio, a screened, roofed structure that would allow your cat to be outdoors without another cat getting to him. If you search for catio information and pictures there are all sorts from very basic to palatial, owner built to commercially available kits - that will give you ideas for the options.

Being mommy to cats given the freedom of the outside, visits from "intruders" into their territory is unavoidable. In my experience, your cat/cats decide whether to allow the strangers access. For those kitties that insist on hanging around or being aggressive towards your cats, I was fortunate having one or two very territorial cats and later a dog who really enforced a "no visitors" policy by shooing the cats away by barking and sometimes chasing (but never attacking).  He kept the neighbor's cats away, although I had one cat who liked making friends which drove my dog crazy. 

Anyway, you can start by NOT allowing other cats inside your house. The outside can become the "neutral grounds". But not the inside (although Gus might not mind). Be sure there is no food left in the open. Shooing them away when they appear helps. I hiss, clap loudly and sometimes wave a large item like a tablecloth or tshirt  (but it  also scared my cats so I had to do a lot of comforting and apologizing afterwards). Oh, and it is better to catch them off guard. If they get startled real good they'll be discouraged from coming back.

Hope my post helped. 

On occasions when the cats have made it into the house they have been shooed away very quickly and  ive only seen one repeat offender actually try to come back in. Although i was told last night that there was a random grey cat in the house who when seen scampered off pretty fast. We think he was in to get food which means he made it through the living room and into the kitchen undetected! 
Gus was a rescue cat so when we first got him he had a litter tray which for reasons only known to him he continues to use. I was thinking it could be this that's attracting that cats in as it would be smell of another cat, which would then also make them come into the garden?