Looking at NMEA Revealed,

We have for description of the field altitude:

Antenna Altitude above/below mean-sea-level (geoid) (in meters)

And for description of the field geoidal separation:

Geoidal separation, the difference between the WGS-84 earth ellipsoid and mean-sea-level (geoid), "-" means mean-sea-level below ellipsoid

Let's note (as in answer by @mkennedy):

h = height above ellipsoid
H = elevation, orthometric height (i.e. antenna altitude in GPGGA)
N = geoidal separation (some books call this the geoidal height)

And consider N to be negative. Then according to catdb.org, the mean-see-level is below elipsoid ("-"), so H > h. So H = h + D with D positive. As N is negative we must take D = - N.

So the correct formula is:

h = N + H

Looking at NMEA Revealed,

We have for description of the field altitude:

Antenna Altitude above/below mean-sea-level (geoid) (in meters)

And for description of the field geoidal separation:

Geoidal separation, the difference between the WGS-84 earth ellipsoid and mean-sea-level (geoid), "-" means mean-sea-level below ellipsoid

Let's note (as in answer by @mkennedy):

h = height above ellipsoid
H = elevation, orthometric height (i.e. antenna altitude in GPGGA)
N = geoidal separation (some books call this the geoidal height)

And consider N to be negative. Then according to catdb.org, the mean-see-level is below ellipsoid ("-"), so H > h (H is height from mean see level, h is height from ellipsoid). So H = h + D with D positive. As N is negative we must take D = - N.

So the correct formula is:

h = N + H