Gusta and Gustan are both Spanish words and are action words, i.e., Spanish verbs. Although one of the two words is singular while the other is plural, the main difference between the two is that both words can't be used exactly the same way. To further explain this, the singular verb of the two is "Gusta," while the plural of the two is "Gustan."
The two words are similar in meaning to the word "like" in English, although the word "like" cannot completely capture their meaning. In short, what you should think of as their first difference is that one is a singular verb, while the other is a plural verb. The understanding of this basic fact will help you to construct a correct sentence. Also, know that "Gustan" should be used when you a plural subject in a sentence, while "Gusta" should be used with a singular subject.