How do we prevent gases from entering the mold cavity in terms of casting design or gating system, i.e., the ingate should be smaller?

Warning

The following answer supposed this question to be about injection moulding.

Frankly speaking, I don't get the question. Noone can prevent gases to be present in the mould, unless one operates the mould in vacuum or suck off the air from it before injection.

Under normal condition, the material inlet is rarely source of gases. The most common occurences would be caused by insufficient backpressure, too big decompression or humidity in material. In the worst case scenario, one may encounter gas coming from decomposed material, but honestly this is terribly wrong set machine.

In general, one cannot prevent gases being there. One just needs to be sure to let the gas evacuate while the mould is filled.

To make ingate smaller is hardly good idea as smaller nozzle increase your injection pressure.