Somewhat. They don't all need super specific types for every method they've defined, but Sorbet does at least need to know all the classes, modules, and constants in use in your codebase, whether those come from code you've written or code inside gems.

But there's tooling (first-party and third-party) that will either download or generate RBI files defining constants that come from gems. `srb init` is the first party solution, and Shopify's `tapioca` gem is the most popular third-party solution[1].

Unfortunately, because Ruby doesn't have import statements at the top of every file, Sorbet can't just do something like silently treat unknown imports as not having a type (like TypeScript and Flow can do), because then it would never be able to tell between "exists but unknown" vs "typo; does not exist" for constant definitions. This definitely makes the adoption process a little tricker compared to other languages, but it's generally a one-time thing once you've got the tooling set up.

Also if you're ever having trouble getting the tooling to work, there's a lot of people chatting about Sorbet daily at https://sorbet.org/slack

[1] https://github.com/Shopify/tapioca