Learn Spanish (Latin America): Tal vez el gentío agobia a los turistas.

Tal vez (like quizá) can take either the indicative or the subjunctive depending on the speaker's level of certainty. By using the indicative (agobia), the speaker expresses a strong suspicion or high certainty that the crowd actually does overwhelm the tourists. If they felt it was less likely, they would use the subjunctive (agobie).

What is the difference between gentío and gente?

Both refer to people, but gente is the general word for "people," while gentío specifically means a "crowd," "throng," or a dense mass of people. It emphasizes the overwhelming number or physical closeness of the group.

Why do we need the a in a los turistas?

This is the "personal a." In Spanish, when the direct object of a verb is a specific person or group of people, it must be preceded by a. Since the tourists (los turistas) are the people receiving the action of being overwhelmed, the a is required here.

How would I apply this lesson's exclamation structure to say "How the crowd overwhelms the tourists!"?

You would use the ¡lo que + verb! structure to emphasize the extreme degree of the action. It would become: ¡Lo que agobia el gentío a los turistas!