1. User provisioning
To log in using SSO, you need your users to exist in the Atlassian product. There are several ways to achieve this and you can read more about the different user provisioning options.
2. SSO protocol
We currently only support OIDC protocol for GitLab integration, but also support SAML for other identity providers. Here you can read about the difference between SAML and OIDC (OpenID Connect).
Go to the setup guide for OIDC.