Acceleration due to gravity is 0 at the center of Earth.
g' = g r / R
mass of Earth = M = 4π/3 R³ * d , where R = radius and d = density
g = G M / R² using Newtons gravity equation.
= G 4π/3 R³ * d /R² = G 4π/3 d R
At a radius r < R, the mass that contributes to gravity is only that contained inside the sphere of radius r. The spherical shell between radii r to R, does not contribute. This is because the attraction forces from all directions will cancel one another.
g' = G M' / r² = G (4π/3 r³ d) / r² = G 4π/3 * r d
g'/g = r/R
so when r = 0, g' = 0