Users of Garmin GTN and GNS navigators may now use the GPS CDI as an alternate means of guidance along the final approach course of a VOR or NDB approach, provided they monitor the ground-based navaid to ensure that they’re tracking the proper final approach course. Previous editions of the AFM supplement for GTN and GNS avionics required you to display the VOR CDI on your HSI or PFD even if you could monitor the ground-based navaid on a separate CDI or by using a bearing pointer.
Note that you must still display the VOR/LOC (“green needles”) CDI to fly the final approach segment of an approach based on a localizer or any other type of navaid except a VOR or NDB.
For more information about setting the CDI while flying approaches, see
Setting the CDI on a Conventional Approach (The “Kill Switch”). For general background, see Use of Suitable Area Navigation (RNAV) Systems on Conventional Procedures and Routes
The updated language in the AFM supplement for the GTN and GNS series (see below) synchronizes the limitations in the AFM supplement with a 2016 update to AIM 1−2−3. Use of Suitable Area Navigation (RNAV) Systems on Conventional Procedures and Routes (see Use of IFR GPS on Conventional Approaches).
In March 2020, Garmin also updated the AFM supplements for the GNS 530W and GNS 430W to reflect this change. See the 190-00357-03_g version of that document, available at the Garmin website.
The change came with a recent update to the system software for the GTN line of GPS navigators (more information about the new features at BruceAir here).
The new software brings a significant change to the language in the approved Airplane Flight Manual Supplement for the GTN boxes (the PDF of the new AFM supplement for the GTN 750 is available here).
Section 2.10 Instrument Approaches in that AFM supplement now notes the following:
…c) The navigation equipment required to join and fly an instrument approach procedure is indicated by the title of the procedure and notes on the IAP chart. Navigating the final approach segment (that segment from the final approach fix to the missed approach point) of an ILS, LOC, LOC-BC, LDA, SDF, MLS, VOR, TACAN approach, or any other type of approach not approved for GPS, is not authorized with GPS navigation guidance. GPS guidance can only be used for approach procedures with GPS or RNAV in the procedure title. When using the Garmin LOC/GS receivers to fly the final approach segment, LOC/GS navigation data must be selected and presented on the CDI of the pilot flying. When using the VOR or ADF receiver to fly the final approach segment of a VOR or NDB approach, GPS may be the selected navigation source so long as the VOR or NDB station is operational and the signal is monitored for final approach segment alignment. [Emphasis added]
A test of the new software in the free Garmin PC-based trainer indicates that the message warning the pilot to switch the CDI from GPS to VOR has also been removed. The following captures show the VOR-A approach at Paine Field (KPAE) north of Seattle flown with the CDI with GPS selected. Note the cyan bearing pointer behind the magenta GPS CDI.
BruceAir, LLC 
Bruce,
Do you know if the Supplement for the 530W has also been amended?
Thanks,
Craig
I doubt that Garmin will update the AFM supplements for discontinued units like the GNS series. But you can check the product pages at the Garmin website. I’ll go out on a limb here and suggest that given the language in the AIM and the new GTN supplements, the FAA is OK with pilots using this technique with other units, as long as they observe the restrictions imposed in the new AFM supplement for the GTN and the AIM.
Garmin has updated the GNS 530W AFM supplement (190-00357-03 Rev. G) to allow use of GNSS along the final approach course:
“Revised section 2.6 to reflect updated AIM guidance related to CDI source selection on
approach”