Hello Yuri
Sorry for not being more specific, and initially misunderstanding your intent. Using the IOS 15.X implementation of GRE + IPSec, it is not possible to more granularly select traffic to be encrypted beyond using the following access list:
access-list 100 permit gre any any
Specifying particular IP addresses (loopbacks) will not allow you to selectively encrypt traffic. You either encrypt all or non of the GRE traffic. If you want to do this you must use the configuration described in the previous post.
However, you must ask yourself, what is the purpose of doing this? If you are not running routing protocols, then a VTI or IPSec tunnel mode would be the easiest way to go. If you are running routing protocols, it is not necessary, nor functional, to just encrypt traffic between loopbacks.
From my understanding, your question is more due to curiosity, and that’s great as it helps us to learn. But is there a practical reason for which you wanted to implement something like this?
I hope this has been helpful!
Laz