I thought gustar was modified by the thing being liked (vs the person/people doing the liking), but when I chose gustan for liking the horse races I was marked wrong.
If what you're saying it that you like "something" and that something is plural, yes, you need "gustan":
Me gustan las carreras de caballos.
But here, the sentence is expressing that they like "to do / doing something", with an infinitive. In this case you need "gusta":
Me gusta ir a las carreras de caballos.
I like to go to the horse races.
I hope it clarified it.
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