Security Gateway uses certificates and becomes an intermediary between the client computer and the secure web site.
Here is a lot of useful information Best Practices - HTTPS Inspection.
HTTPS Inspection - Inbound vs. Outbound
- Outbound HTTPS Inspection protects internal users and perimeter servers from malicious attacks coming from the Internet on connections originated from inside the organization.
- Inbound HTTPS Inspection protects internal servers (for example, data centers and web servers) from malicious attacks coming from the Internet.