The below height chart has been circulating around the internet:
But I don't think it's accurate, so I did some rough calculations using a different source.
The circulated chart puts Gamagori at 2.69m or 8" 8'
KillLaKill.com have some character profiles on their site of the different characters.
If we assume Ryuko has the average female Japanese height - 158 cm (5 ft 2 in), then we could estimate Gamagori's height by resizing the images so that the scale of the characters is more reasonable. I used head size for this, as the differences should be smaller than if we compared other features:
1.58m * 1.66 Ryukos = 2.6228m or 8"7'. This is quite similar to the above image
There are obviously lots of flaws in this calculation. Scaling was based off of visual inspection, Ryuko may not be the average female height, not using the standard Nonon unit of measurement, etc. But it does give a certain range that his height may fall in.