GPS accuracy is not country-dependent, since it is a space-based system. *I've
never heard of GPS certification for countries; can you point me to a typical
NOTAM for this?

I am quoting from Angle of Attacks comprehensive video guide on the
FMC's POS REF page.
They state that the 'GPS NAV (ENABLE) button (Line Select 5 Right)
toggles the GPS position (default is on) 'as some countries do not
meet certain standards for GPS navigation', 'the GPS will then try to
correct an accurate IRS position with an inaccurate GPS position,
knowing what countries this happens in are part of the NOTAMS that a
pilot would get in his pre-flight with dispatch'. I wasn't aware of
this either so it would need looking into if you're curious