The CMS Physical, Occupational & Speech Therapy Billing Guide states (page 68) that those who furnish outpatient therapy must add specialty-specific modifiers too. These are:
GN — Services delivered under an outpatient SLP plan of care.
GO — Services delivered under an outpatient OT plan of care.
GP — Services delivered under an outpatient PT plan of care.
Big picture: There are 42 functional G-codes, comprising 14 sets of three codes each. Generally, six of the G-code sets are used for PT and OT functional limitations and eight of the G-code sets are for SLP functional limitations.