I think there is no bigger loss for a child then losing their parents.
First Gayu was deserted by her real father and then she came to accept her mothers remarriage.
The parents are the pillars of strength for a child as they are growing up. Those pillars help to build their confidence and sense of security. She lost that when her parents passed away so she is missing that sense of security. She is seeking validation of love and acceptance from the family. She now considers Naitik and Akshara her parents as they have raised her now. She may not be their real child but I would say she is their adopted child in a way. She is now part of the family.
Just because she is not Akshara's child by birth it is wrong to say she is not part of the family.
When you say that you are showing difference and treating her like a stepchild which will make her insecure. A child that has lost their real parents always has an emptiness in their heart which is hard to fill. To compensate for that pain she is getting fulfillment by love and acceptance of Naitik and Akhara, Naira, and Naksh. Also her need for a loving relationship is to fill that void and she sees that void being filled by Karthik. I think the comments made about Gayu here are a bit harsh. Sometimes the child who is not a child by birth does more for the parents then ones own real child. To say because she is not their child by natural birth so she is not part of the family is really wrong and only someone with life's experience would understand that. (another parent) To say she should live like a true orphan is very insensitive, selfish, and really shows lack of compassion for other people. People adopt kids everyday. Do those parents love their child any less? No way. Look at Nandini. How much she loves Yash who is her adopted son. He does more for the family then their real son. '
The situation really happens in real life and to say a child who has lost their parents has no right to belong to a true family and feel fully loved and accepted is plain wrong. Writers are showing a real sensitive real life situation that people face. Elders in the show are saying right when they say Gayu shouldn't feel any difference because she is also part of the family. That is why they told Naksh that give same to both Naira and Gayu for Raksha Bandhan.