Hello, I am questioning about how was this pagoda getting called as the Big Goose Pagoda.

As for the reason why it is called Big Wild Goose Pagoda, there is a legend. According to ancient stories of Buddhists, there were two branches, for one of which eating meat was not a taboo. One day, they couldn't find meat to buy. Upon seeing a group of big wild geese flying by, a monk said to himself: 'Today we have no meat. I hope the merciful Bodhisattva will give us some.' At that very moment, the leading wild goose broke its wings and fell to the ground. All the monks were startled and believed that Bodhisattva showed his spirit to order them to be more pious. They established a pagoda where the wild goose fell and stopped eating meat. Hence it got the name.

isn't this is what you are questioning for? it has been written in the article.