At my school, if you do 2 3/4s in year 11, you are forced by school policy to take 7 subjects in year 11 (because we must do 5 in year 12). If you're bright enough to do two 3/4s in year 11, you should be capable of coping with 7 subjects. Plus, GMA is a fairly awesome subject (if you like maths), and even then you don't need to worry about pressure because you don't have plans to continue on to Spesh. But, in saying that, in the case that one of your 3/4s don't turn out as you'd hoped, you can always take up spesh.
You need GMA for your uni prereq, so there's not a lot to discuss unless you're willing to give up your uni course. So basically, I'm just reassuring you in that you should be fine with your subjects. I did 7 last year and I seemed to cope. And especially at my school, having spares is just a waste of time.