Long time user and admin, first time deploying this through AWS marketplace subscription.
Deployment went fine, no snags, go to the EIP and the login screen shows up fine but no matter what default credentials I use (root, admin etc etc) using the known default passwords: instance ID, IP, 5l!f one etc etc
Nothing works.
And the doc is less than helpful.
Please help 
You can change root password yourself.
gitlab-rails console
# find root user
user = User.where(id: 1).first
# modify your password
user.password = 'secret_pass'
user.password_confirmation = 'secret_pass'
# save your password
user.save!
In addition to the steps to reset password using Rails console that @Jie shared, there is also a Rake task to reset passwords in GitLab 13.9 and newer.