In what portion a non-GPS approach, can a pilot legally fly with GPS as primary navigational resource? In IFR-GPS-equipped (/G) aircraft, a pilot flies most of the segments with GPS as primary navigational resource* even the route is along victor airways or VOR radials. However, when we start the approach, we have to switch the nav source from GPS to the nav source specified for the approach. In this context of using GPS for non-GPS approaches, 


When should a pilot switch away from GPS? Is it from an IAF, FAF or upon ATC vectors to intercept the final approach course?
If the a pilot executes a missed approach, when can the pilot switch back to GPS? Is it when established on course direct to the holding fix?




*By primary navigational resource, I mean it provides the signal/data source for the CDI needle on a HSI, that a pilot, autopilot or flight director refers to make actions or action suggestions.
An appropriately installed GPS may be used until crossing the final approach fix on a non-localizer approach, and until the segment requiring navigation via the localizer on a LOC, ILS, SDF, or LDA approach. It's generally approved and legal at any other time.

During the missed approach, a pilot may switch to GPS immediately, unless the missed approach requires navigating by using the localizer, in which case the ground-based navaid must continue to be used until the procedure no longer requires navigation by reference to the localizer.

The FAA covers this in AC 90-108:


  An otherwise 
  suitable RNAV system cannot be used for the following:
  a.   NOTAMed   Procedures. 
  Unless otherwise specified, 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.
  b.   Substitution on a Final Approach Segment. 
  Substitution for the NAVAID (for 
  example, a VOR or NDB) providing lateral 
  guidance for the final approach segment.
  c. Lateral Navigation on LOC-Based Courses.
   Lateral navigation on LOC-based courses 
  (including LOC back-course guidance) 
  without reference to raw LOC data.