The gharara is a popular bridal dress for Muslim brides in the Hindi belt(UP, Bihar etc) but during the nawab era in the Awadh region, it was a common piece of clothing for the women in the region, usually among the aristocrats
The gharara became a significant part of Indian Muslim culture because it was most often worn by Muslim women at the time.
When the partition happened, one of the concerns of the leaders of Pakistan was the assimilation of people into a uniformed culture.
And since the gharara was already associated with Muslim identity, it seemed like the ideal choice to help create a cultural identity for the women of Pakistan.
Influential women like Fatima Jinnah a.k.a Madar-e-Millat(mother of the nation) and Begum Rana Liaquat Ali Khan would wear the gharara often which helped the people of Pakistan identify with the garment as an aspect of Pakistani culture