In this paper, we discuss the methodology of height datum unification by means of the Geodetic Boundary Value Problem (GBVP) approach and tide gauge information. We apply the global multiple vertical datum GBVP approach with an observation equation for the datum offset written in terms of the ellipsoidal height of the mean sea level at a tide gauge, the height of mean sea level in the national vertical datum and the geoid height. An example is given for CGVD28 and NAVD88 datums in North America. A few issues related to the geoid height are studied: the magnitude of the so-called indirect bias term, as well as the GOCE global geopotential model (GGM) commission and omission errors and their effect on the accuracy of the computed datum offsets. It is shown that the indirect bias term is below 1 cm if a residual Stokes’s kernel is used, which corresponds to a degree and order 180 of the GOCE GGM. The GOCE geoid commission error computed from the time-wise approach GGM of degree 180 is 2–3 cm at the North American tide gauges. The GOCE GGM omission error could affect the computed mean vertical datum offsets at the tide gauges by as much as 7 cm.