Question

Why is the gangetic plain also called the northern plain

Answer 1

I believe the question will be in reverse as the northern plain is called the gangetic plain
here's why:
When the Himalayas were lifted up about 2400 kms from Jammu and Kashmir to ArunachalPradesh a deep valley was formed along the mountains this valley later on filled with the sediments brought by the river Ganga and formed present Northern Plain so, we call it as the Gangetic plain.

Answer 2

This is bcoZ it is located in the northern part of the country at the foothills of the great himalayas