Others have explained, but I found it funny to read this word. My native language is Dutch (flemish variety) and I would also read this word as expressing a negative emotion (fed up sounds right). "gat" = dialect/slang for ass, "vol" = full.
It disappoints me that so many fellow South Africans are seemingly oblivious to the audience they are interacting with and casually use local slang regardless.
When I saw it I guessed it was some local slang, and I was excited to learn what it meant from someone who really knew rather than some online dictionary. I don't think it should be a disappointment, it contributes to the wonderful diversity of language.
Reading South African as a Dutch person is always great. I understand most of the words, but it’s still clearly a completely different language/context.
As a non-South African, it's always great to learn local slang from around the world. On forums such as HN or Reddit, there's always someone around to explain it means. It's never an impediment to understanding the point being made.
I’m not South African, my husband is. I picked it up from him. We use it around our home quite a bit. I was replying to a South African. Whilst talking about South Africa. I'm fairly sure nobody's upset about it, more intrigued.