On a double-swing gate, the drop bolt secures the 'inactive' leaf so the active leaf has a stable target to latch against. On a single swing, it stops the leading edge sagging or being pried open. Common drop-bolt sizes are 12 to 20 mm diameter. The ground sleeve is a length of pipe set in concrete with a closeable cap that keeps dirt out.
Placement follows wherever the leaf sits most stable when shut, usually 50 to 100 mm in from the leading edge. The ground sleeve has to be deep enough that the bolt can't be lifted out by an attacker. Plan on 50 mm of engagement minimum for any security-rated install.
CAD60 swing-gate drawings include drop-bolt positions on the front elevation and a detail of the ground sleeve.