Google Cloud Identity Management is a good, and most importantly free, choice because it provides comprehensive security, seamless integration with Google cloud services, single sign-on capabilities, scalability, customisation and cost-effectiveness.
However, I can note that it will be difficult to figure it out on your own.
An alternative to Google Cloud Identity Management is Microsoft Azure Active Directory (Azure AD).
Also, for a complete understanding, I advise you to read about Azure Cost Optimization
Good luck!