The headline feature of the 0.4 series will be adding support for an alternative mechanism to fetch, at build time, dependencies that are not vendored into the gb project.
This mechanism is opt in only and is activated by the presence of a file at the root of your project, tentatively named $PROJECT/depfile. The depfile contains the missing information required to download a specific release version of a library from upstream.
gb 0.4.0 lays most of the groundwork for implementing this feature, but not the feature itself.
Please consult issue #536 for full details.