GPS use is not universally approved for all types of Navigation... very similar to the fact that you can't use GPS in order to calibrate your VOR receiver even though it definitely would work...
you must either do a VOT test or cross check between dual receivers...if you did; this will be the applicable reg*. ---assuming you work for a US carrier---and even if it is approved on the chart .... your OpSpecs are regulatory--and are the company's medium for part 121 compliance!
I'm not being a wise *ss
*91.13 Careless or reckless operation
(a) Aircraft operations for the purpose of air navigation. No person may operate an aircraft in a careless or reckless manner so as to endanger the life or property of another.
(b) Aircraft operations other than for the purpose of air navigation. No person may operate an aircraft, other than for the purpose of air navigation, on any part of the surface of an airport used by aircraft for air commerce (including areas used by those aircraft for receiving or discharging persons or cargo), in a careless or reckless manner so as to endanger the life or property of another.