A top roller, or guide roller, is a small nylon or polyurethane idler fixed to a guide post beside the opening. A pair of them, one each side of the top rail, keeps the gate from tipping over as it slides. They guide the gate; they do not carry it.
On a tracked sliding gate the weight sits on the bottom rollers and the track. On a cantilever gate the weight sits on the roller carriages that run inside the cantilever rail under the gate. Some trade literature calls those carriages rollers too, but they are a different part doing a different job.
Guide rollers are sold in pairs on an adjustable bracket, sized to the thickness of the gate.