In terms of the Byakuya knocks Gerard to the ground and knocks off Gerard's helmet.
It's more along the lines of Byakuya is actually disintegrating Gerard's body up to where his severed head comes off, hence Hinamori's disgust and shock at how they went overboard to make sure an enemy stayed dead. On top of this, remember it is plausible due to Gerard's ability. Whatever is severed and removed grows back bigger and stronger.
180cm is actually 5ft9" so the height measurement is wrong
This is incorrect. 180cm is 5.9 FEET but feet are not measured using tenths or one hundredths as a Foot is not divided evenly by 10 or 100 to get inches. There are 12 inches in a foot so 180cm translates to 5.9 FEET but this is 5 ft and 11 inches (or 10 inches and 55/64 but that's rounded up). I suggest you not use Google to calculate that. Here. I suggest you learn the difference between the imperial and metric system.