The LOC-D approach for SAN DIEGO/GILLESPIE FIELD (SEE) shows different
minimums for "GRIGG FM".

Yup.

There is an ident code shown for GRIGG, but no frequency.

Frequency for all marker beacons is 75 kHz (or is it MHz - never can
remember) - in any case, you either have a marker beacon receiver (most
are built into audio panels these days), or you don't. If you do, it's
the right frequency.

Is the dot-dash-dot ident just the generic MM ident?

Nope.

I can't find "GRIGG" listed as an airspace fix at airnav.com. Are the
higher minimums to be used for an approach in an aircraft without an
operating audible marker annunciation?

Not sure audible annunciation is required if there is visual
annunciation, but I've never seen a marker beacon receiver that didn't
have audio (whereas a lot of the old ones didn't have visual). In any
case, the higher mins are for aircraft that can't identify GRIGG.

The interesting question is this - can an alternative method (like IFR
GPS) be used to identify GRIGG? In any sane world, the answer would be
yes. If GRIGG was a compass locator (meaning an NDB was collocated
with the marker beacon) the answer would be yes. As it is, it may
legally be no. I know that IFR GPS can substitute for ADF or DME on an
approach (other than an NDB approach with no overlay) but I've never
seen any guidance that indicates it can substitute for a marker beacon.

What does "FM" stand for?

Fan marker. They used to exist on low frequency airways, once upon a
time. They're no longer described in the AIM, as far as I can tell.

Michael