Yes, It is required to install a plinth beam between two columns, which used for installing an iron gate. The reason to provide it is when the vehicle comes closer to the iron gate and passes through the gate, a load of the vehicle is acting on that beam, and the beam will be able to resist the load. If we can't install beam between two columns, the land depressed at that location and this may impact your gate and if soil bearing capacity is not enough then maybe iron gate collapse.

Wilma Caldwell
Architectural technologist