Question:

According what calendar will be prefer for Bnei Noah to celebrate their birthday – Jewish or Gregorian?

Answer: 

There is no specific commandment to celebrate the date of birth, that’s why we can’t answer this question from the point of vew “what God wants from us”.

If Ben Noah has relatieves, with whom he wants to celebrate his birthday, and for them the Gregorian date is more habitual, than he can celebrate the birthday according the Gregorian date.