If the noun is both feminine and masculine, then what would be the definite article?? For example, for
gente, it is both feminine and masculine, so what would the definite article be??
There is no “generic gender” in Italian: every name is either masculine or feminine. The word gente is feminine, so it wants the article la even when you're referring to a group of male people.
Another instance is persona: if you're talking about a male, you have to use the article la (that is, la persona) nonetheless.
Words like gente or persona don't have gender variations like figlio and figlia, so for them the feminine article will always be used. If a word has gender variation, when talking about “mixed” situations the masculine form is used: Giacomo ha quattro figli, due maschi e due femmine.
A few words can receive both articles because the masculine and feminine forms share the word; you find some of them at this article on Treccani. Going on with the example above we would say Giacomo ha salutato i suoi quattro nipoti even if some of them are male and some female.