Todo, todos, todas and toda are in this case being used as adjectives, meaning that they have to agree with the noun it is describing. You would say "toda la gente" to mean "everyone" instead of "toda gente". "Gente" is another word for "people"; however, "gente" is a singular noun and thus, it needs a singular adjective. "Personas" is plural, so the adjective describing it needs to be plural.