GitLab leverages Google's reCAPTCHA to protect against spam and abuse. GitLab displays the CAPTCHA form on the sign-up page to confirm that a real user, not a bot, is attempting to create an account.
To use reCAPTCHA, first you must create a site and private key.
Go to the URL: https://www.google.com/recaptcha/admin
Fill out the form necessary to obtain reCAPTCHA keys.
Login to your GitLab server, with administrator credentials.
Go to Applications Settings on Admin Area (admin/application_settings)
Fill all recaptcha fields with keys from previous steps
Check the Enable reCAPTCHA checkbox
Save the configuration.