Hello Stephen ,
Te gusta la chica is another way of saying "la chica te gusta " so the conjunction of the verb "gustar" refers to the "la chica" , the 3rd personal pronoun (she) .
Te gustan las chicas is referring to " las chicas " , the personal pronoun ( they) ,so the verb's conjunction follows the noun in both cases .

This phrase literally (somewhat) means "something (third person) pleases you". See it here: "te (you) gusta (it pleases)". However, "gustan" would refer to something third person plural. Therefore it would say "they (something third person plural) please you". Here is the alternative way of seeing it: "te (you) gustan (they please)".