How to deal with GPA-related questions in standardized job application forms, if you have no GPA for graduate studies
As noted in the comments, the first thing to do is to contact the company HR department.  

If you don't get a rapid response, however, then I would recommend filling in the form with as obvious a nonsense number as it will let you (e.g., -1, 9000), or if it won't let you do that, then fill in whatever it thinks is a perfect GPA (you passed all your classes, right?).  Then explain the form problem and your true situation in text.  The best place to do this is, in order of preference:


A free-text "Comments" or "Other Information" field on the application form, since that will typically be seen at the same time a person is seeing the GPA.
Your cover letter
Your resume


Note, however, that you are not very likely to get a positive response in any case from this type of completely generic application form, since it is likely getting filled to the brim with pseudo-spam from a vast number of recent graduates worldwide.  HR departments often put such a "catch-all" form out in order to satisfy formal requirements, rather than because they expect to actually do much hiring in this manner.