To export things from GitHub, the proper authorizations must be set. This can be done using GitHub integration with OAuth registration, where GitLab authenticates with GitHub to access a list of projects for the user.
The best result of mapping authors and assignees from GitHub to GitLab is achieved when using the GitHub integration features that are available in GitLab instead of using a single personal access token from GitHub.
The integration covers all the import items we just mentioned and tries to keep all the references intact. For instance, the importer will try to find the GitHub authors and assignees of issues and pull requests. This means that GitLab integration has to be enabled in your GitLab instance.