Owners and Maintainers of the Project will receive an email notification from GitLab 3 months prior to deletion.  To prevent the Project from being deleted, you can take any action that adds an entry to the Project's activity log.

For more on finding the date of a Project's last activity, see https://docs.gitlab.com/ee/administration/inactive_project_deletion.html#determine-when-a-project-was-last-active.