Did Jagya lie to her before getting married? Before going in to the registrar's office, did Gauri not know that Jagya was a married man? Jagya might've lied to her before, but when they got married, Gauri knew very well that he was a married man. So now that she's made her bed, she must lie on it.
She thought it was all very simple, "... anyway Anandi is destined to live the life of a deserted wife, so why should I care for it, if I can be happy with her husband" that's what she told her mother before getting married.
When she got the legal certificate, she thought she had stolen a march on the family that had rejected her, and waved that in front of them, "I am the legal wife of Jagat, so you had better accept me". When she saw that they believed not the legal marriage but the child-marriage as the legal one, immediately she changed her tune, "I too was married to this boy as a child, so you had better accept me".
Gauri is getting her just desserts from the family, and nothing but the humiliation she's herself inviting at every step. She has to take responsibility for the decisions she has taken and no one is to blame for her own actions.