Represents the "Gamma" entry of the calgray colour space
It is an optional entry defined as part of the PDF 1.0
It contains a double representing a PDF numeric object.
It should have a positive value.
The PDF specification states that this item assumes a value of 1
if no value has been provided.
For definitive details see:.
The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 63,
The ISO PDF
Specification, ISO 32000-2:2017 PDF 2.0; Table: 62, page 183.