Learn Polish: Przed sceną zgromadziło się aż tysiąc osób. - As many as a thousand people gathered in front of the stage.

The word aż is an emphatic particle that shows surprise at how large the number is. Without it, the sentence is just a neutral fact: "A thousand people gathered." By adding aż, you express that a thousand is an unexpectedly or impressively high turnout, which translates well to "As many as a thousand."

Why is the past tense verb zgromadziło się in the neuter singular form rather than plural?

In Polish, when the subject is a numeral of five or higher (like tysiąc), the past tense verb must take the third-person neuter singular form. Even though the logical meaning is plural, grammatically it acts like "a quantity of a thousand [it] gathered."

Why is the word for people in the form osób?

Osób is the genitive plural of osoba (person). Whenever you use a numeral of five or more in the nominative or accusative case, the noun being counted must be in the genitive plural. It literally translates to "a thousand of people."

Could I use the imperfective verb gromadziło się instead?

You could, but it would change the nuance. The perfective zgromadziło się shows a completed action: the crowd is fully formed and the final headcount is a thousand. Using the imperfective gromadziło się would mean they "were gathering" over time, which doesn't fit as naturally with the sudden surprise of the total amount emphasized by aż.