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There is a difference between substitute means of navigation and alternate means of navigation.

Again from the AIM:
a. Discussion. This paragraph sets forth policy concerning the operational use of RNAV equipment for the following applications within the National Airspace System (NAS):

1. As a substitute means of navigation guidance when a VOR, NDB, DME, or compass locator facility is out-of-service (that is, the navaid information is not available); an aircraft is not equipped with conventional equipment such as ADF or DME; or the conventional equipment such as ADF or DME on an aircraft is not operational. For example, if equipped with a suitable RNAV system, a pilot might hold over an out-of-service NDB.

2. As an alternate means for navigation guidance when a VOR, NDB, DME, or compass locator facility is operational, such that the pilot can revert to the underlying guidance, as necessary, but does not normally monitor the underlying aid. For example, if equipped with a suitable RNAV system, a pilot might fly a procedure or route based on operational VOR using RNAV equipment but not monitor the VOR.

Further in the section it states:

c. Allowable Operations. Subject to the requirements in this paragraph, operators may use an RNAV system for the following operations:

1. Determine aircraft position over a VOR, NDB, compass locator, or DME fix.

2. Determine the aircraft position over a named fix defined by a VOR course, NDB bearing, or compass locator bearing crossing a VOR or localizer course.

3. Navigate to or from a VOR, NDB, or compass locator. For example, a pilot might proceed direct to a VOR or navigate on a segment of a departure procedure. However, pilots may not substitute for the navigation aid 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.

4. Hold over a VOR, NDB, compass locator, or DME fix.

5. Fly a DME arc.

These allowances do not include navigation on localizer-based courses (including localizer back-course guidance).

This is the same intent from AC 90-108:
8b. Substitution on a Final Approach Segment. Substitution for the NAVAID (for example, a VOR or NDB) providing lateral guidance for the final approach segment.

8c. Lateral Navigation on LOC-Based Courses. Lateral navigation on LOC-based courses (including LOC back-course guidance) without reference to raw LOC data.


From this is were Boeing derives their guidance for raw data requirements. As an example, this is from the 747 FCTM:

Raw Data Monitoring Requirements:
During localizer-based approaches; LOC, LOC-BC, LDA, SDF, and IGS, applicable raw data must be monitored throughout the approach.

During non-localizer based approaches where the FMC is used for course or path tracking (VOR, TACAN, NDB, RNAV, GPS, etc.), monitoring raw data is recommended, if available. For airplanes with two operational FMCs, two IRSs and two GPS receivers (or two DME receivers if GPS updating is not available), or if the FMC is RNP/ANP capable, raw data monitoring is not required.


For VOR, NDB etc. approaches without GPS in the title, you can use GPS as an alternate means of navigation for the final segment.

But you can't use it as a substitute means of navigation for the final segment, unless the operator has Ops Spec C300 approval.