The bnconfig tool is run every time the machine is rebooted to configure the hostname again. In case you run it manually and configured a domain, you can rename that bnconfig tool to not to run it anymore.
sudo mv /opt/bitnami/apps/gitlab/bnconfig /opt/bitnami/apps/gitlab/bnconfig.disabled
This guide has a warning message in case you configured a domain in the application
Let us know if that solves the issue.
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