The steps you will need to do in SmartConsole in order to get the connection working behind the Internet Security Gateway are:

Define an accept rule in Security Policy. This rule allows the traffic from your internal networks to pass through the Security Gateway.

Define automatic NAT for each network to NAT the networks behind a public IP. This option translates the private IP addresses of your internal networks to a public IP address assigned by your ISP router. This way, your internal networks can communicate with the Internet using a valid IP address.

Publish and install the policy. This step applies the changes you made to the Security Gateway and activates the security and NAT rules.

References: Check Point R81 Quantum Security Gateway Guide