“Hay quien cree/piensa...” is used informally.
You can say it with out “gente” due to verbs creer/pensar, that are referred only to people, gives you the idea you are talking about they. This is called “omitted subject” in Spanish.
But pay attention, if you omit this the subject, it’s better to say “quienes” instead of “quien” because you aren’t talking about one person but people.
Does it helps you?