I failed my test as well (really failed it with high numbers) and I, like you, was very concerned. However, it really is manageable. I would recommend eating lots of veggies; drink plenty of milk, and some protein and not worry too much about failing the test.
They say you can have more of a chance of delivering a large baby due to the increase in glucose but my daughter was a petite but healthy 5lb 2oz. They will also check at your next sono to see how your fluid levels are since moms with gestational diabetes tend to have higher fluid levels in the womb. My fluid levels were normal the entire time so yours might very well be normal as well.
After delivery they checked my daughter's insulin levels and they were normal and a few weeks after I delivered my numbers went back to normal as well and they have stayed normal for over a year. So my point is that even though you fail the test, you won't necessarily have all of the "possible" complications associated with gestational diabetes.
Enjoy your last couple of months! You are almost there!