According to sk89980: Support for Sub-interfaces / Alias IP address / Secondary IP address in Check Point product..., support of Alias / Secondary IP is very limited in GAiA:
Gaia OS supports the configuration of Secondary IP addresses only on single Security Gateways. In all other releases/configurations (e.g., ClusterXL, Dynamic Routing), the use of Secondary IP addresses is not officially supported. If the physical machine does not have enough physical interfaces, then VLAN interfaces should be configured.
So i do not think that this is supported on GAiA Embedded without these limitations - if supported at all, maybe the sk105380 is rather speaking of VLANs...
CCSP - CCSE / CCTE / CTPS / CCME / CCSM Elite / SMB Specialist