The height of a gantry is measured from High Rail Level (HRL) to the lower edge of the gantry.
Please note, the height of some gantries is limited by the associated mast. For example, sometimes gantry height can only be adjusted in increments compatible with the mast height increments available.
Gantry Height - Single Track Cantilevers
Gantry Height - Portal
More info: Learn how to adjust gantry height.