On a cantilever gate, the top roller assembly is the structural fulcrum. The cantilever rail rides between two pairs of rollers (one pair near the latch side, one pair further up the gate) bolted into a heavy bracket on the receive post. The gate's full weight pulls down on these rollers; the counterweight tail balances the moment.
Top rollers usually come as a packaged kit from sliding-gate hardware suppliers, sized for the rail profile (e.g. 80x80, 100x60) and rated for a gate weight (commonly 400 to 800 kg). The rollers are nylon or polyurethane wheels on sealed steel bearings, set in a steel housing that grips both faces of the rail.
On a tracked sliding gate, the term 'top roller' or 'guide roller' refers instead to a small idler at the top of the gate that just keeps it from tipping over. Same name, different job.