A concreted post is set in a hole excavated in the ground, typically 500–900 mm deep and 300–450 mm diameter, then filled with concrete around the post. The concrete spreads bending and overturning loads into a wide soil footprint. For a residential single swing-gate post: a 100×100 SHS in a 900 mm × 400 mm dia. concrete footing is the common spec.
Concrete cure time is at least 24 hours before the gate can be hung; full design strength is reached at 28 days. Posts must be plumb in two directions before the concrete sets, usually braced with timber struts.
CAD60 lets you specify post depth per post and surfaces the concrete depth in the drawing pack alongside post material and mounting type.