A gate is a small opening

A gate is a small opening (or orifice) through which the poly melt enters the cavity. Gate design for a particular application includes selection of the gate type, dimensions and location.
The gate design is largely determined by:

The part geometry (wall thickness etc, )
Part specifications (appearance, tolerances etc-)
Material used
Fillers used
Cycle time
De-gating requirements

Unless it is necessary to use multiple gates (e.g., the length of melt flow exceeds practical limits), a single gate is generally preferred Multiple gates always create problems of weld and meld lines. The cross-section of the gate is typically smaller than that of the runner and the part, so that the part can be easily de-gated (separated from the moulding Gate thickness is usually two-thirds of the part thickness.
Technician speaking, the material freezing off at the gate indicates the end of the cavity packing phase. Hence there is no point in maintaining pressure after this point is reached.This in turn means the gate must be large enough to make sure that the molding is properly filled before gate freezing occurs.
A larger gate dimension will reduce viscous (frictional) hearing, permit lower velocities, and allow the application of high packing pressure to increase the density of the material in the cavity. If low stress is a requirement, owing to demand of aesthetic appearance or dimensional stability, a larger gate may be necessary.