yes mandatory to file return , income is computed before deductions and this case you have to file mandatory

Yes... Its taxable Income

gift received from relative is not taxable, right?
still have to file?

Sum of money received without consideration by an individual or HUF is charged to tax if the aggregate value of such sum received during the year exceeds Rs. 50,000. Once the aggregate value of monetary gift received during the year exceeds Rs. 50,000, then the aggregate value of gift received during the year will be charged to tax.​

yes. but the section also states -

"Provided that this clause shall not apply to any sum of money received—

 (a)  from any relative; or

 (b)  on the occasion of the marriage of the individual; or

 (c)  under a will or by way of inheritance; or

 (d)  in contemplation of death of the payer; or

 (e)  from any local authority as defined in the Explanation to clause (20) of section 10; or

 (f)  from any fund or foundation or university or other educational institution or hospital or other medical institution or any trust or institution referred to in clause (23C) of section 10; or

 (g)  from any trust or institution registered under section 12AA."

what is the inference?

Gift received from relatives are exempt from tax. Who will be considered as relative for the purpose of claiming such exemption?

Following persons would be considered as relative ​

(a) Spouse of the individual;

(b) Brother or sister of the individual;

(c) Brother or sister of the spouse of the individual;

(d) Brother or sister of either of the parents of the individual;

(e) Any lineal ascendant or descendent of the individual;

(f) Any lineal ascendant or descendent of the spouse of the individual;

(g) Spouse of the persons referred to in (b) to (f).​

Gift received only on the occasion of marriage of the individual is not charged to tax. Apart from marriage there is no other occasion in which gift received by an individual is not charged to tax. Hence, gift received on occasions like birthday, anniversary, etc. will be charged to tax.​​

 

exactly.
He has received gift from his grandfather (lineal descendant). so its exempt from tax right?