I ran into the same issues, what you need to do is drop the existing database, createa new one, then run the database restore:
dropdb -h /var/opt/gitlab/postgresql/ mattermost_production
createdb -h /var/opt/gitlab/postgresql/ mattermost_production
gunzip -c ~/mattermost_dbdump*.gz | psql -h /var/opt/gitlab/postgresql mattermost_production