Problem

Protobuf-generated sources are not resolved as Generated Sources:

and cannot be used in a different Gradle module. An error Cannot Resolve Symbol is shown

Solution

1. Add the idea plugin to a Protobuf module build.gradle file with the configuration below:

sourceSets.main.java.srcDir new File(buildDir, 'generated')
idea {
  module {
    // Marks the already(!) added srcDir as "generated"
    generatedSourceDirs += file('build/generated')
  }
} 

specified paths may vary depending on a project configuration.

2. Add implementation project(':protobuf') dependency to a build.gradle file of a module where Protobug-generated sources are used. Ensure that a dependency is visible under File | Project Structure | Modules | module_name | Dependencies

3. Hit the Reload All Gradle Projects button in the Gradle tool window: