A gate stop is a piece of steel (commonly a 50 mm length of angle iron, a short post, or a welded block) that physically halts the swing of a gate at a chosen point. Open stops keep the gate from swinging past 90 degrees and tearing the hinges. Closed stops back up the latch and stop the gate over-swinging past closed before the catch plate engages.
Closed stops live on the latch post or in the ground beneath the leading edge. Open stops live in the ground at the end of the swing arc, padded with rubber so the gate doesn't ring like a bell on impact.
Low-cost, often overlooked, but the stop is the difference between a gate that lasts twenty years and a gate whose hinge welds crack in three.