First create a backup of your stack just in case :
sudo /opt/bitnami/ctlscript.sh stop
cd /opt
sudo cp -a bitnami bitnami_backup
sudo /opt/bitnami/ctlscript.sh start
After that just run the following to reset the user's password :
cd /opt/bitnami/apps/gitlab/htdocs
echo 'u=User.find_by_id(1); u.username="NEW_USERNAME"; u.save!;' | RAILS_ENV=production /opt/bitnami/ruby/bin/ruby /opt/bitnami/apps/gitlab/htdocs/bin/rails c
echo 'u=User.find_by_id(1); u.password="NEW_PASSWORD"; u.save!;' | RAILS_ENV=production /opt/bitnami/ruby/bin/ruby /opt/bitnami/apps/gitlab/htdocs/bin/rails c