Add ChatGPT OAuth provider

What does this MR do and why?

https://gitlab.com/gitlab-org/developer-relations/contributor-success/team-task/-/issues/1211

Adds ChatGPT as an OmniAuth OAuth2 provider, allowing users to sign in to GitLab using their OpenAI/ChatGPT account.

This includes:

• A custom OmniAuth strategy (OmniAuth::Strategies::Chatgpt) implementing OAuth2 + PKCE against OpenAI's auth endpoints
• User identity is extracted from the OIDC id_token JWT payload (the sub, name, email, and email_verified claims)
• Sign-in button icon for the ChatGPT provider
• Provider label mapping (chatgpt → ChatGPT)
• Example configuration in gitlab.yml.example

Configuration

Admins can enable the provider by adding the following to their OmniAuth configuration:

omniauth:
  providers:
    - { name: 'chatgpt',
        app_id: 'YOUR_APP_ID',
        app_secret: 'YOUR_APP_SECRET' }

The provider supports auto_link_user (matching existing users by email) and allow_single_sign_on (auto-creating new accounts) as with other OAuth providers.

References

Screenshots or screen recordings

How to set up and validate locally

1. Add the ChatGPT provider to your config/gitlab.yml under development.omniauth.providers:

   development:
     omniauth:
       providers:
         - { name: 'chatgpt',
             app_id: 'YOUR_APP_ID',
             app_secret: 'YOUR_APP_SECRET' }

2. Obtain the client_id and client_secret from the Engineering 1Password Vault: ChatGPT Dev OAuth Application
3. Restart GDK
4. Visit the sign-in page and click the ChatGPT sign-in button
5. Authenticate with your OpenAI account
6. Verify you are signed in to GitLab

Test auto-linking, by setting auto_link_user: true at the omniauth: level and ensure a GitLab user exists with the same email as your OpenAI account.

Test SSO by setting allow_single_sign_on: ['chatgpt'] at the omniauth: level and make sure a user DOES NOT exist with the same email as your OpenAI account.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

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