For Gayatri, it is as though someone has just said a complete stranger is her son. I know it seems like she should have a connection, but that doesn't happen automatically. My personal opinion is that she is too afraid of losing the son she does see as her own, Yash, to think beyond that. You can't really think straight when you are overcome by such a fear and once Gayatri is assured that Yash will always be her son, she may be able to look past that and think about Akash. Right now everyone is talking as though it has to be either or, and Gayatri has a strong emotional connection to and need for Yash, so she is clinging to him at all costs, ignoring all other truths. It's not that she is pushing Akash away, he is just not in her line of vision because she is consumed by the fear of losing her favourite son.
Radha's case is a bit different. She has always known Yash was her biological child, and in fact has ministered the whole situation for his happiness. So that investment has always been there. Also, for some reason, Radha is confident that she will never lose Akash, which I am not sure she should be. She is taking him for granted while keeping Yash's priorities higher and he will notice and react now that he knows the truth.
I think it is an interesting contrast: Gayatri who is afraid of losing Yash and finds that the cost of keeping Yash as her son is to forget about Akash vs. Radha who is over-confident that she can get rights for both her sons and get both their love and respect. I think both of their assumptions are going to be challenged.