May be they are showing Gangaur festival because they have shown all the other festivals like Basant Panchmi, Holi, Diwali Teej and what else😛
Just looked up the net about Gangaur festival. It is right now being celebrated with great pomp in Rajasthan. 😆
So they decided to show their drama around this festival for a change.
It looks like all girls irrespective of the fact whether the are married or not celebrate this festival do vrat.
Since it involves acrobats like carrying pots on head Nandini may not have been made to do this when she was small.
As for observing Harki doing this all these years - don't think the girl is in the least bit interested in Harki Devi and her life when it doesn't have anything to do with her.😛 So it is possible that she didn't know the finer details other than keeping tab on the fact that they all got to wear new clothes😆