We can't calculate automatically the value of the properties of <w:gridCol/> tag, it takes a value when it's displayed in a document or saved by them. The problem is that Google fails on that tag of the OOXML standard:
OpenOffice fails too when reading some tags of the standard.
PHPDocX is a library to generate DOCX documents according to a standard and we must apply this OOXML standard and not solve every miss and errors of the viewers.
Of course as PHPDocX is an open library, you can change the code to fit your needs.
Thank you for your understanding.