I suggest you start testing for a while, eg morning (fasting), 2 hr after each meal, and at night. Then take these readings to your GP, to discuss stopping the gliclazide (the metformin could be increased maybe, can't remember what the max dose is but it's 2000mg or thereabouts). Then if you do decide to stop the gliclazide, you can continue testing for a while, to make sure your blood sugars don't rise without the gliclazide.
I'm sure gliclazide is less harmful than high blood sugar btw.