In reference to the OP’s original question about VOR and NDB procedures:
Yes for a Missed Approach Segment or Hold.
NO for a Final Approach Segment WITH LATERAL GUIDANCE.
The answer as it pertains to the final approach segment is NO if the Navaid is providing lateral guidance. And, you do not have to monitor the underlying Navaid EXCEPT for the final approach segment.
In other words, you can fly to, from, and around a Navaid without tuning in to the Navaid. You can even use GPS to fly to the runway while monitoring the Navaid. As soon as you are using Lateral guidance to align the plane with the runway for landing purposes, you have to use your Nav radio.
I imagine that the only time a VOR would provide lateral guidance by scaling would be if the VOR were located on the field. This is probably making a distinction between VOR/DME and NDB approaches not providing alignment guidance, and all other approaches. Probably since a VOR not located on the field will point you to the runway. But, it will not funnel or lead/guide you down to it (scaling).
And, your database must be current and contain the desired procedure to legally use the GPS.
See the below excerpt from the latest FAR/AIM as of 26 Jan 20. Specifically AIM 1-2-3 Notes section 2 & 4.
The FAR/AIM says:
The allowances described in this section apply even when a facility is identified as required on a procedure (for example, “Note ADF required”).
These operations do not include lateral navigation on localizer-based courses (including localizer back-course guidance) without reference to raw localizer data.
Unless otherwise specified, a suitable RNAV system cannot be used for navigation on procedures that are identified as not authorized (“NA”) without exception by a NOTAM. For example, an operator may not use a RNAV system to navigate on a procedure affected by an expired or unsatisfactory flight inspection, or a procedure that is based upon a recently decommissioned NAVAID.
Pilots may not substitute for the NAVAID (for example, a VOR or NDB) providing lateral guidance for the final approach segment. This restriction does not refer to instrument approach procedures with “or GPS” in the title when using GPS or WAAS. These allowances do not apply to procedures that are identified as not authorized (NA) without exception by a NOTAM, as other conditions may still exist and result in a procedure not being available. For example, these allowances do not apply to a procedure associated with an expired or unsatisfactory flight inspection, or is based upon a recently decommissioned NAVAID.
Use of a suitable RNAV system as a means to navigate on the final approach segment of an instrument approach procedure based on a VOR, TACAN or NDB signal, is allowable. The underlying NAVAID must be operational and the NAVAID monitored for final segment course alignment.
For the purpose of paragraph c, “VOR” includes VOR, VOR/DME, and VORTAC facilities and “compass locator” includes locator outer marker and locator middle marker.
d. Alternate Airport Considerations. For the purposes of flight planning, any required alternate airport must have an available instrument approach procedure that does not require the use of GPS. This restriction includes conducting a conventional approach at the alternate airport using a substitute means of navigation that is based upon the use of GPS. For example, these restrictions would apply when planning to use GPS equipment as a substitute means of navigation for an out-of-service VOR that supports an ILS missed approach procedure at an alternate airport. In this case, some other approach not reliant upon the use of GPS must be available. This restriction does not apply to RNAV systems using TSO-C145/-C146 WAAS equipment. For further WAAS guidance, see paragraph 1-1-18.