I don't have the answer either sadly but I too am going through something similar and feel for you both Harleys and Mouse.
I am 27 weeks today (and age 41) and two weeks ago I had a private scan where baby boy's tummy and head were measuring off the charts big - beyond 100th centile . The annoying sonographer said I 'almost certainly have GD' and must get tested immediately. She said there was almost no chance that it was simply a big healthy thriving boy (even though All my baby's measurements are above the 80th centile, I myself weighed 10lbs at birth and my husband is a big guy).
In a complete state I called my consultant (I am very lucky to be going private at Lindo Wing). He said I could do a GTT immediately which I did ten days ago. It came back normal but only just. My blood glucose level two hours after drinking all that revolting Lucozade was 7.2 (the 'normal' cut off is 7.8).
Saw consultant the next day and he asked me to REPEAT the test this week at 27 weeks as that is his preferred earliest time to do the GTT anyway. So I did it again yesterday (more belching and burping) and the results came back exactly the same. Actually 7.3 after two hours so slightly worse.
I am feeling very grumpy about this as he clearly is not able to definitively tell me whether I should be worrying about this.
Appears consultant will continue to monitor me and scans and I predict more GTT tests coming my way as the weeks pass.
A very large baby isn't too big an issue as I am booked to have a planned c-section in late May when I will be 38.5 weeks (my first son was a c-section so happy to make things simple with another). But clearly if I am going to develop GD before that then it IS an issue which needs to be managed for my baby's sake.
I have booked another scan at the Fetal Medicine Centre (my favourite place for scans) for next week at 28 weeks to see how the measurements are doing.
My sonographer at the FMC (who I also rang after the 'bad scan' which was somewhere else) said that not only do they look for very large tummies in GD babies, but also excess fluid (which you mention above), but she ALSO said that the tummy doesn't only measure big, it also has an unmistakable 'fatty quality' to the scan picture which she and her colleagues at FMC are absolutely trained to recognise.
Dunno if any of this helps but I would love to stay on here with you if I may as having had HG for the first half of my pregnancy I could really do without this crap now and need someone to talk to about it!