Des gens se sont groupés sur la place = (some) people have gathered on the square
Je vois des gens de ma fenêtre = I can see (some) people from my window.
Le peuple est souverain = The people is sovereign
Les peuples d'Europe sont souverains = The peoples of Europe are sovereign.
In French, "gens" is always masculine/plural, but you can use "les" or "des" depending on the sentence's construction.
"Le peuple" can be singular or plural, and represents the population of a given country/land.
In English, "people" with no article is just a group of several persons (from a few to a crowd).
In French, all nationalities have a noun and an adjective, whereas in English, some nationalities having no substantive form, "people" is also used with a nationality adjective (les/des Japonais = Japanese people).
beaucoup de gens; il n'y a pas de gens ici; les gens ici sont gentils; les gens sont venus au musée / les peuples autochtones d'Afrique
gens is for people in general, peuple is specific to: 4. the entire body of persons who constitute a community, tribe, nation, or other group by virtue of a common culture, history, religion, or the like: the people of Australia; the Jewish people. 5. the persons of any particular group, company, or number (sometimes used in combination): the people of a parish; educated people; salespeople. 7. the subjects, followers, or subordinates of a ruler, leader, employer, etc.: the king and his people. http://dictionary.reference.com/browse/people