That's interesting, because none of the built-in (predefined) IPSec or L2TP 'services' (read: Port Forwarding Rules) had GRE enabled. (see last screenshot in my post).
However, I can add GRE to the new 'service' I defined in Advanced > Port Forwarding Rules.
I did that this evening, saved the rule, re-verified the port fowarding rules under Firewall Settings > Port Forwarding now shows GRE.
With GRE enabled in the 'all-in-one' L2TP/IPSec rule, its now allowing me to connect to the VPN server behind the Actiontec router.
Many thanks for that tip! :smileyhappy: