"I like it" in English puts the person doing the liking in the subject spot. Gustar flips that around: the thing being liked is the subject, and the person is just along for the ride as an indirect object. That's why gustar almost always shows up as gusta or gustan, agreeing with whatever's being liked instead of with the person.
Gusta agrees with one thing being liked, or an infinitive. Gustan agrees with more than one -- the verb never conjugates for the person doing the liking.
One thing (or an infinitive) is doing the pleasing
More than one thing is doing the pleasing
A ella le gusta el libro, pero a mí me gustan las películas basadas en él.
She likes the book, but I like the movies based on it.
Gustar never conjugates for the person doing the liking -- me/te/le/nos/les just marks who's affected. The verb itself always agrees with the thing being liked: gusta for one, gustan for several.
Take the quiz and see if you can match gusta and gustan to what's being liked.