Is it possible to implement GRE over IPSec on a WatchGuard Firebox? And how can it be configured?
Review this article:
Configure a BOVPN virtual interface in Fireware v11.11.x that uses GRE for dynamic routing to a Cisco endpointhttps://techsearch.watchguard.com/KB?type=Article&SFDCID=kA10H000000g3AQSAY&lang=en_US
The linked document does refer to a quite old version of Fireware (11.11.x), but the concept seems to be the same.
One thing I have noticed is that if you select the "Remote Endpoint Type" as "Firebox" based on the tooltip that shows in Policy Manager, this gives you a GRE over IPsec tunnel.