For example, some implementations require multicast traffic to be encapsulated before IPSec encrypts it. If this is a requirement for your environment and the GRE tunnel and IPSec tunnel share the same IP address, Add GRE Encapsulation when you set up the IPSec tunnel.
How to configure gre over ipsec？
How to configure?
In that same document but in the next section is a whole area on configuring GRE:
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