Learn French: Des millions de gens regardent ce film. - Millions of people watch this movie.

In French, numbers like million and milliard (billion) are actually nouns, not adjectives. Because they are nouns, they require an article in front of them. Since we are talking about multiple millions, we use the plural indefinite article des.

Why do we use de gens here instead of des gens?

Expressions of quantity in French are almost always followed by a simple de (or d'), without the plural article les. Just like you say beaucoup de gens (a lot of people), you say des millions de gens.

Could I use personnes instead of gens in this sentence?

Yes, des millions de personnes works perfectly too! While gens is generally used for undefined groups of people and personnes is required when you count people with specific small numbers (like trois personnes), large estimates like millions can take either word naturally.

How is the verb regardent pronounced in this sentence?