Every serial has a scene like this. Maayka did. Waaris did. Vivaah did. Frankly, I find it very silly.
I understand this is a traditional symbol of marriage, but to reduce the meaning of marriage down to this one gesture is insulting to the whole institution. Amar put sindoor on Gauri. But that's ALL he did. It's not as though he consummated the marriage. There is so much to his and Divya's relationship that does not and never will exist between him and Gauri. DIvya is pregnant with his child, for crying out loud. Does this one act of desperation really undo all of that?
There is a lot more to a wedding and to marriage than this one gesture, and that's all it is--a gesture. I guess in the west, putting a ring on someone's finger would be a similar tradition. Does that mean that when I go shopping for jewelry, and the salesman helps to place a ring on my finger, that I am married to him? Of course not.
Marriage is a spiritual and legal bond. No such thing exists between Amar and Gauri. I agree it was kind of a silly way to "save" her, just like it was a silly way for Jeet to save Soni and a silly way for Raj to save Trishna, etc. It always seems to be a matter of saving the damsel in distress in these situations. But in the end, I do not find it meaningful at all and I think the reaction everyone is having is overly dramatic.