When an interface acquires its IP address dynamically, such as with dynamic-PPPoE, that interface can be selected as the IPSec Gateway. Even if the interface is selected, though, the IP address remains "None" because it is not possible to select the IP address.
The configuration process selects the interface and leaves the IP address set to none. The dataplane automatically uses the dynamically assigned IP address. The IP address cannot be selected because that would lock the configuration to reference a specific IP value. If the interface receives a new IP address during the next PPPoE negotiation, the configuration will become invalid and the VPN tunnel will break.