To use the equipment suffix "G", does the GPS have to be certified for IFR? To use the equipment suffix "G", does the GPS have to be certified for IFR flight?
Yes, using GPS for IFR requires a certified unit. The AIM 1-1-17(b)(2) says (emphasis mine):


  Authorization to conduct any GPS operation under IFR requires:
  
  (1) GPS navigation equipment used for IFR operations must be approved in
  accordance with the requirements specified in Technical Standard Order
  (TSO) TSO-C129(), TSO-C196(), TSO-C145(), or TSO-C146(), and the
  installation must be done in accordance with Advisory Circular AC
  20-138, Airworthiness Approval of Positioning and Navigation Systems.
  Equipment approved in accordance with TSO-C115a does not meet the
  requirements of TSO-C129. Visual flight rules (VFR) and hand-held
  GPS systems are not authorized for IFR navigation, instrument
  approaches, or as a principal instrument flight reference.