Hi there,
In that log, you are trying to (make us) connect to the git provider with https and a login/password; we do not support that.
You would indeed need to generate one key per netlify site (not one per repo!) to automate connecting our site to services via an API. This article explains the steps you’ll need, though it leaves you to build the flow you’ll need to access GitLab to get a key, etc:
Once you get it right, you’ll see that we no longer throw errors like the one you quote:
...could not read Username for 'https://gitlab.com': No such ... since deploy keys will not fail with that sort of error, even if they are wrong.