Plate is flat steel, distinguished from sheet by thickness. The convention in Australia is that anything 5 mm or thicker is plate; anything thinner is sheet. Common plate thicknesses for gate work are 6, 8, 10, 12, 16, 20, and 25 mm.
Plate is the universal source for footplates (cut a 200x200x10 plate, drill the bolt-hole pattern, weld to the post), hinge plates (cut a 100x100x6 plate, weld to the stile, bolt the hinge to it), gussets (cut a triangular plate, weld it into the corner of a frame for stiffness), and reinforcement washers behind any high-load bolt.
Mild-steel plate is grade 250 or 300 to AS/NZS 3678. Higher-strength plate (350, 400, 450 grade) costs more but lets you use thinner plate for the same load.