In FEMAP the term for "g-force" is Body Load. It's entered under "Loads" in the model tree.
Recall that FEMAP is unitless. Be sure your units for Body Loading is consistent with the rest of your model.
e.g.: 1 g down = -386 [in/s²] or -9807 [mm/s²] in the "Z" field depending on the units of the model.