The primary function of a gate valve is for creating isolation i.e. on or off. The secondary function of a gate valve can be for flow reduction - where the bore is tapered to be reduced bore.
Gate valves should not be used for throttling (i.e. using the gate in any semi open / closed position) as this will result in erosion of the sealing surface. Gate valves which have been used for throttling will generally show a wear pattern on the bottom edge of the gate.
Gate valves are generally used as block valves i.e. either fully open or fully closed. Operation is by lifting / lowering the gate between the two seats, by turning a manual hand wheel or other actuating device attached to the stem.
The turning of the handwheel causes either the stem to rise pulling up the gate, or the gate to screw up the thread of the stem, hence the definition, rising stem or rising gate. The seat to seal interface in a gate valve is created by the gate pushing down between 2 seats - either machined from the body, swaged or inserted in to the main body of the valve.
Energisation of the seat in a wedge gate valve is purely by mechanical force exerted through the stem, by turning the hand wheel. In parallel slide gate valves, energisation of the seat can be by springs between a split gate. Process pressure can also assist in providing a downstream seal in gate valves.