Acceleration due to Gravity at the surface of the Earth

G = UnitConvert[Quantity[1, "GravitationalConstant"], "SIBase"]

ME = PlanetData["Earth", "Mass"]

The Earth is not precisely spherical.    Mathematica reports the Polar Radius, the Equatorial Radius, and the average Radius.

REpole = UnitConvert[PlanetData["Earth", "PolarRadius"], "SI"]

gpole = G ME/REp^2

REeq = UnitConvert[PlanetData["Earth", "EquatorialRadius"], "SI"]

geq = G ME/REeq^2

RE = UnitConvert[PlanetData["Earth", "Radius"], "SI"] (* This is the same as (Polar + Equator)/2. *)

g = G ME/RE^2

v = UnitConvert[2 π REeq/Quantity[1, "SiderealDays"], "SI"]

arad = v^2/REeq

geff = geq - arad