Medicare's MLN Matters Number: MM10176 was recently revised to identify services subject to their therapy cap. The revision became effective on January 1, 2018 and some providers have begun to receive claim rejections because they are not using the appropriate modifier. The article states the following (emphasis added):

Services furnished under the Outpatient Therapy (OPT) services benefit – including Speech Language Pathology (SLP), Occupational Therapy (OT), and Physical Therapy (PT) – are subject to the financial limitations, known as therapy caps, originally required under Section 4541 of the Balanced Budget Act (1997).

There are two such caps. One cap is for PT and SLP services combined and another cap is for OT services. In order to accrue incurred expenses to the correct therapy cap; the use of one of the three therapy modifiers (GN, GO, or GP) is required on a certain set of Healthcare Common Procedure Coding System (HCPCS) codes in order to identify when each OPT service is furnished under an SLP, OT, or PT plan of care, respectively. 

There are some MACs which have required modifier GP in this situation. All of the following codes are on that list:

Since this update is from Medicare, it is likely that your MAC has updated their claim editing process to reflect this update and are now requiring modifier GP on all the above services. Additionally, modifier GP is needed for physical therapy evaluation codes 97161-97164.