I know that USyd takes ATAR over GPA if you've studied for less than a year. I'd say that UNSW would be the same.
The course I applied for at UTS has always been extremely popular for NRSL (Design/Visual Communication) so I guess that's why they have more qualifications for admission. Plus it just depends on the philosophy of the faculty/university and at what point they will deem your most recent study as overriding the study you did in high school. Anyway, obviously I can't answer with 100% certainty, but I'd say that USyd and UNSW would be fine. Just have a look at the university website for the degrees that you're potentially considering transferring into. They will specify on the page whether they'll look at ATAR/GPA, and if they don't, just give the uni a call.
I wouldn't suggest a full gap year, I think it's actually a perfect solution to study for a semester and then take half a semester off. It's 8 months off too, which is pretty substantial. That way you get a taste of university life and see what sort of things you enjoy and don't enjoy. Maybe even consider sitting in on some lectures this semester of the degrees you're considering transferring into, if you don't have too many contact hours at uni.
Anyhow, good luck!