Not everyone is Yash dear, who actually contradicted the fact that blood need NOT necessarily be thicker than blood.
But well this is Gayatri you are talking about. So, she is different.
Actually what I feel is that she does love Yash, but it's just that she is frustrated with the 7 days thing and knowing that Akaash is her son, she does feel something for him too, and maybe that is why she has a preference when it comes to him now.
I guess that is why Mr SP. did NOT want Akaash in SH as he knew that sooner or later, this would have happened, knowing Gayatri's nature.