An end stop is a piece of steel that lives at each end of a sliding gate's range of travel and physically halts the gate. The closed-position end stop sits at the catcher post side and stops the gate over-shooting the latch. The open-position end stop sits at the runout end and stops the gate sliding off its rail entirely (which would be expensive and dangerous).
For a tracked sliding gate, the stops bolt to the V-track or U-track itself. For a cantilever, they bolt to the cantilever rail or to the receive post bracket.
Stops are usually rubber-faced so the gate doesn't ring on impact, and they're sized to take the gate's full inertia: a 400 kg gate moving at 0.2 m/s carries 8 J of energy that needs to dissipate somewhere.