A hydraulic gate closer is a sealed piston, mounted between the gate and the post, with two adjustable speed valves. One valve controls the swing speed across most of the arc. The other controls the final 'latch speed' over the last 10 to 20 degrees, so the gate eases into the latch instead of slamming.
Closers come into their own on driveway gates, school and commercial gates, and any leaf over about 60 kg where a self-closing hinge spring would either fail to close or close hard enough to be unsafe. The Locinox Samson range and CAME series are the common Australian options.
Mounting matters. The closer body usually mounts to the post, the arm reaches across to the gate stile. Allow service room: pistons leak after 5 to 10 years and need replacement, not repair.