An end stop is a block of steel at each end of a sliding gate's travel that stops the gate. One sits at the catcher post end so the gate does not over-run the latch. The other sits at the runout end so the gate cannot slide off its rail, which would be 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.