No seu gitlab.rb:
##
## If allow_username_or_email_login is enabled, GitLab will ignore everything
## after the first '@' in the LDAP username submitted by the user on login.
##
## Example:
## - the user enters '[email protected]' and 'p@ssw0rd' as LDAP credentials;
## - GitLab queries the LDAP server with 'jane.doe' and 'p@ssw0rd'.
##
## If you are using "uid: 'userPrincipalName'" on ActiveDirectory you need to
## disable this setting, because the userPrincipalName contains an '@'.
##
allow_username_or_email_login: false