I did the interview 3 days ago, and they offered me the salary that match my expectation in "Gross" and the "job offer" letter. Should I re-negotiate to get "Net" salary?
There is nothing wrong with re-negotiating your offer letter, if you have new arguments. If nothing has changed since your interview, then I wouldn't suggest it.
E.g. I had the same dilemma before. I got offered what I requested, but in the meantime I have received +20% offer from another company. I told the first company that I really want to work for them, but the salary gap between these two offers is too big and I would like them to increase the initial offer by 15%. They agreed.
All in all, if anything has changed since interview it's always worth to ask.