That is a tough question, I am not totally familiar with medicare billing codes. However according to my research, the GW modifier is used when a provider of medical services performs services that are not related to a hospice diagnosis. Certain Medicare beneficiaries can choose hospice benefits instead of Medicare benefits for treatment and management of their terminal condition.
reference: http://wpsmedicare.com/j5macpartb/resources/modifiers/hospicemodifier.shtml