R81 VSX site to site vpn source interface

[checkpointer]

Hello 

In the following topology, I would like to change the source interface for my site-to-site VPN from my gateway's main public IP address to another interface on the gateway, but I have nothing in the drop-down when I try to do this in the gateway object inside the community. I cannot change the main gateway IP address as this affects other VPNs.

Currently:

(GW main interface) 10.10.10.10 ------------->192.168.10.10 (3rd Party)

But I want:

(GW other interface) 20.20.20.20 ------------->192.168.10.10 (3rd Party)

I try to change my source interface inside VPN community but get nothing in drop-down: