They say (whoever "they" are) that "dogs live in the moment
". Cats do too, but also they do seem to hold grudges, and sometimes for a long time.
I remember an incident when my hubby inadvertently stepped on one of my girl's paws while going downstairs while she was coming up, and he yelled and swore at her as he almost tripped. After that one
incident, she avoided him completely, even runnng out of the room if he came in. This went on for almost 13 years
(and she lived to 17-1/2yrs.), and he would always make some kind of derogatory remark when she did that...like "stupid cat! what did I do?". I finally
convinced him to pay attention to her and try and sweet talk her and pay her compliments. And you what? she started responding to him, talking to him, rubbing up against him, to the point that she became his lap cat
within a short time. So I learned that if you do something to a cat
like step on its tail or paw by accident, you have to apologize immediately
. It's really amazing, because when you don't the cat thinks you hurt it on purpose, but seems to understand an apology (or maybe its the tone of voice?) and doesn't hold a grudge, but just forgets about the accident. At least, this has been my experience with cats.
Seems to me Gizzy is holding a grudge against you because Forest is getting attention, he's jealous and he's being shut out. Can he top it? Yes, he hissed at you and he can start peeing more places and up the ante by more hissing or worse.
Then you'll have an even bigger problem with him than "marking" at times. I hope you can get in his "good books" soon. However, if your love for him has been replaced by hatred,
perhaps a divorce would be better for all concerned and he should be rehomed. Maybe your'e not compatible--it happens. There's no point in keeping a pet where no one likes him, and he's miserable too---something to think about. Do you think he may be happier as an only cat for someone?