You might need to follow these steps:
- Configure GAIA OS to authenticate against your tacacs server using cli or webui
- Configure the site to site vpn between the gateways and make sure that the tacacs server is part of the encryption domain
- The tricky part any traffic matches implied rules will not be encrypted you might do the following:
- In global properties set the "Accepte outgooing packets originating from gateway to be before last"
- if the above option doesn't work you might need to to disable tacacs in implied_rules.def file on the mgmt server and create rule o allow the firewall to access the tacacs server and push policy
Thanks