I have seen in some portals that mail I'd can't be corrected

Bunny Myeon You can change gender, category ,parents details now all this comes under personal details in the same page where the email Id is. Now if they don't allow it then contact them in their helpline no. But email id is important as its used for contacting you by gate though they use both mobile no and email iD. Send them email let them clarify what they will allow and will not allow to change and whether there is any solution. Try to be precise and clear.

Bunny Myeon There are only two feasible explanation if they are not allowing to change email ID either its not that important a detail then gender,paper change,exam city change or it didn't come to them that candidates would do error in entering email ID.

Vidya Kasi https://engineering.careers360.com/articles/gate-application-form-correction
Check this link

Bunny Myeon Contact them today itself https://gate.iitkgp.ac.in/contact_us.html they may take time to respond