What does an industry recruiter want to know my Ph.D. GPA for? What are the potential motives for an industry recruiter to want to know your PhD GPA? 

Does this imply the position is more of an office drone position than a pure research position?
The most likely motive is your recruiter believes PhD GPAs are as relevant as undergrad GPAs.  My guess is this recruiter has a plethora of office jobs and doesn't usually work with academics.

It is perfectly acceptable to tell the recruiter you're not interested in working with them