Garbage collection automatically reduces the capacity marketed in the product description.
actually, both spindle disk and SSDs do that.
Here is how it works, in computers, GB = 1024 MB, MB = 1024KB, KB = 1024B. This is because everything is base 2... its actually 2^X.
Companies that sell drives are using the scientific definition of kilo, mega, and giga... namely, giga = a billion.
So intel sells you a drive with 120 billion bytes (120GB), the computer however is using the computer giga and the conversion is to take 120 / 1.024^3 = 111.76GB.
And those drives do have reserve space, the drives use chips whose size is actually measured in computer giga. The reserved space is the difference between the two, aka 1.024^3-1^3/1^3 *100% = 7.3741824%
Note that they do not actually advertise that space. They advertise it as 120GB (scientific) ignoring the extra 7.37% reserve space (which is not due to garbage collection)
Anyways, it has nothing to do with garbage collection and it happens in ALL drives, INCLUDING spindle drives.