SHS is the square sibling of RHS, also made to AS/NZS 1163 grade C350. Common sizes for gates and fences are 25x25, 30x30, 40x40, 50x50, 65x65, 75x75, and 100x100 mm with wall thicknesses 1.6 to 4.0 mm.
SHS is the default for gate posts. The symmetric cross-section resists bending equally in both directions, which matters when a post is carrying both swing-gate hinge load and ground-line wind moment at right angles to each other. A 100x100x4.0 SHS post in 600 mm of concrete typically supports a 1.5 m wide x 2.0 m tall single-leaf swing gate.
CAD60's gate models specify SHS post sizes alongside RHS frame sizes. Both are independent inputs.