[Everglow.]

The hissing and growling thing is probably a territorial thing. When I had a stray and my cat was alive she would hiss and growl at the stray because she was invading her territory. It may be out of character for her around humans, but this other cat is technically invading her space, and so she is showing these signs to warn threats off basically. It's what animals do so don't worry too much about that! If anything it shows she can stand her ground.
I'd definitely keep shooing the cat away if you don't want it in your garden though, over time this cat will come to associate the garden with being shooed and hopefully that will cut down how often it visits!