You don't, if you only download. No username, no password needed.
I clearly remember my git clone was rejected before registering. Perhaps this only works with HTTPS?
The url you mentioned, looks like an url for an ssh client. And ssh requires you to auth.
If you clone anonymously, you don't need ssh. What benefit would it give you?.
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Note
git@gitlab.com:freepascal.org/lazarus/lazarus.git
is not a "git://" url. It is a "user@server:path" for ssh.
EDIT
On the other hand, given that they give you a generic username "git", I wonder if maybe it allows anonymous. I did not test.