The exact value of the grand unification energy (if grand unification is indeed realized in nature) depends on the precise physics present at shorter distance scales not yet explored by experiments.
Some Grand Unified Theories (GUTs) can predict the grand unification energy but, usually, with large uncertainties due to model dependent details such as the choice of the gauge group, the Higgs sector, the matter content or further free parameters.
In theory, at such short distances, gravity becomes comparable in strength to the other three forces of nature known to date.
This statement is modified if there exist additional dimensions of space at intermediate scales.
In this case, the strength of gravitational interactions increases faster at smaller distances and the energy scale at which all known forces of nature unify can be considerably lower.