Oh ok. Initially you said "GS intercept at published altitude... nothing
else" identifies the FAF. (Although I'd still be reluctant to rely on a VFR
GPS for identifying a fix.)

Well, my experience with both VFR and IFR GPS leads me to believe that
the difference is basically the blessing of a federal bureaucrat.

Legally, the GS intercept at published altitude, nothing else,
identifies the FAF on an ILS. There is no requirement to check the
altimetry or anything else. Of course if it's not a big deal to do so,
the prudent pilot will. A VFR GPS makes a fine check without adding
workload. I would suggest that this is good enough, and there's no
reason to make things more difficult by flying the LOC inbound by dead
reckoning while checking the cross fix if you don't have to.

Michael