Back to the OP…yes TJ, I think you should go. Most of us have somewhere in our circle of “friends” and “acquaintances” one person who is what we would have termed growing up at home “a fookin eejit”, who behaves appallingly at social occasions and lets him/herself down, denies all knowledge the next day (I never believe that one either) and swans along to the next car crash of an occasion.
It is I’s leaving do and as S won’t be there, at least you won’t be cringing at the thought of another similar event. Also, you said yourself you’ll have a good time and there will be lots of people you know there – plenty of reasons to attend. The only reason you have not to attend seems to be that you feel hypocritical for accepting G’s hospitality in the knowledge that you’re not planning to reciprocate with your own anytime soon. This shows that you are a man of principals, which we all know..nothing new there fella. But there’s always a time when you just have to swallow and do the lesser of two evils…which in this case seems to be going to wish I bon voyage rather than steadfastly sticking to your guns on G’s behaviour.
I sense that you already really want to go for I’s sake. Looks like most here think you should too. It would be best to make the point to G another time.