Caroline, I don't see why your midwife would test for GD because of measuring 1 week ahead. I mean, fundal height in general is not an exact measurement - it's considered "normal" to be within +/- 2cm, I think. So even measuring 1cm higher than your week is still completely normal.
And I've been testing my glucose levels at home (not taking the GTT), and I've been completely normal, despite measuring 1cm ahead last prenatal.
But I know I grow bigger-than-average babies (not really "big" babies). So I'm not at all surprised to measure 1 week ahead by fundal height. I mean, if you look at the father of these babies, you can see why these babies might be big.