Dear Agony Uncle, I have a question.
I had lunch with my girlfriend near her office and everyone in the restaurant who wearing business attire. I don’t do a job requiring to wear a suit or tie. Shall I fit in one next time?
Thank you very much.

The considerable thing to do is to ask your girlfriend. If she is embarrassed at all by your appearing without a tie while being surrounded by everybody else who wears one. If she dose not care, then you must not worry in the slightest. But if she were to be conscious of your standing out, then you should either suggest lunching elsewhere or dumping her for her shallowness.