I am working on my GFG too....I have most of the flowers ready and am now working on sewing on the sashing hexies....
My thought on the ones along the top and botttom : For the "inner" was to create a paper hexie with three normal sides, two longer sides and then another bottom straight side... and then rectangles to get the same length for the "outer" ones in the same length...so that I end up with a straight border. (kind of like your picture but with less extra on the sides...and using some type of shaped paper stabilizer....)
For the sides, I was thinking about the same process as the inner, but two hexie sides and then the rest would end up as a rectangle.... (very much like you did in the first picture on the bottom right)
But I have not mathematically worked it on paper yet. Nor have I thought about the corners.....
But I do like what you are doing with leaving the extra!