Sale of apartment

Since X received the property/flat from his mother through registered gift deed and became absolute owner of flat thus being an absolute owner of property X has every right to sell/transfer the flat to anyone and he need not to take any NOC from his mother.

He don't need NOC from builder. 

Yes.  It is not necessary from the builder.

The builder has got nothing to do with the flat, especially after completion of the construction and handed over possession to the owner/buyer. 

It is his absolute and own property hence he do not have to obtain NOC from anyone to sell the flat, however if there is a registered assocation then he may have to obtain NOC  from the residents welfare assocation.

Thanks for valuable opinions

Very well advised by learned senior experts, I endorse and appreciate their acumen.

How and where is the question / doubt to obtain NOC from builder for the property acquired by way of gift ? 

In the instant case you are absolute owner of the property and there is no role, interest, claim, title or possession of builder, accordingly you can dispose it of to any one of your desire.