A lever-handle latch is the door-knob equivalent for an outdoor steel gate. Pressing the lever (or rotating a ball handle) retracts a spring-loaded tongue that engages a strike plate or catch plate on the post. Letting go re-extends the tongue.
These are the standard pedestrian-gate latch above pool-compliance grade. Locinox makes a stainless-steel range that handles weather and salt air. The mortise-style version (set into a welded lockbox) gives you a key-lock cylinder for entry control.
CAD60's gate models surface a lock-type field with options for lever handle, ball handle, magnetic latch, or no lock. Picking lever or ball handle then offers an 'add lockbox' toggle to weld a steel housing onto the latch stile.