"Everything is free!"
Translation:Alles is gratis!
Technically they are not actually adjectives/nouns. Alle is an indefinite cardinal number and alles is an indefinite pronoun. This is important in that they may sometimes behave differently, e.g. taking different inflections. But alle is in fact used very much like an adjective and alles very much like a noun.
"De winkel geeft sokken weg, alle zijn gratis". (the store is giving away socks, all are free)
"Alle" means "all" and is used for objects. "Allen" means all people. They refer back to things/people that were mentioned before. It might be better to keep these separate from "alles" as the translation is quite straightforward.
- alle(n) zijn - all are
- alles is - everything is